diff --git a/src/proto_alpha/lib_protocol/alpha_context.mli b/src/proto_alpha/lib_protocol/alpha_context.mli
index 585fcce1d9f3a13b3708e5d4f0d922d0cb75f8eb..bfa87a9c92fdce8113f1a2b46b0414fd49d38779 100644
--- a/src/proto_alpha/lib_protocol/alpha_context.mli
+++ b/src/proto_alpha/lib_protocol/alpha_context.mli
@@ -2372,7 +2372,8 @@ module Delegate : sig
     context -> public_key_hash -> Cycle.t tzresult Lwt.t
 
   module Consensus_key : sig
-    val check_not_tz4 : Signature.public_key -> unit tzresult
+    val check_not_tz4 :
+      Operation_repr.consensus_key_kind -> Signature.public_key -> unit tzresult
 
     val active_pubkey :
       context -> public_key_hash -> Consensus_key.pk tzresult Lwt.t
diff --git a/src/proto_alpha/lib_protocol/delegate_consensus_key.ml b/src/proto_alpha/lib_protocol/delegate_consensus_key.ml
index af07fa07ffddc811bc3a80a20e5ad77d1da47137..a8220c2e955bed92d6d217cb9f83162b466d486a 100644
--- a/src/proto_alpha/lib_protocol/delegate_consensus_key.ml
+++ b/src/proto_alpha/lib_protocol/delegate_consensus_key.ml
@@ -26,9 +26,12 @@
 type error +=
   | Invalid_consensus_key_update_noop of
       (Cycle_repr.t * Operation_repr.consensus_key_kind)
-  | Invalid_consensus_key_update_active of Signature.Public_key_hash.t
-  | Invalid_consensus_key_update_tz4 of Bls.Public_key.t
-  | Invalid_consensus_key_update_another_delegate of Signature.Public_key_hash.t
+  | Invalid_consensus_key_update_active of
+      (Signature.Public_key_hash.t * Operation_repr.consensus_key_kind)
+  | Invalid_consensus_key_update_tz4 of
+      (Bls.Public_key.t * Operation_repr.consensus_key_kind)
+  | Invalid_consensus_key_update_another_delegate of
+      (Signature.Public_key_hash.t * Operation_repr.consensus_key_kind)
 
 let () =
   register_error_kind
@@ -54,50 +57,62 @@ let () =
     `Permanent
     ~id:"delegate.consensus_key.active"
     ~title:"Active consensus key"
-    ~description:
-      "The delegate consensus key is already used by another delegate"
-    ~pp:(fun ppf pkh ->
+    ~description:"The delegate consensus key is already used"
+    ~pp:(fun ppf (pkh, kind) ->
       Format.fprintf
         ppf
-        "The delegate consensus key %a is already used by another delegate"
+        "The delegate %a key %a is already used as another consensus or \
+         companion key"
+        Operation_repr.pp_consensus_key_kind
+        kind
         Signature.Public_key_hash.pp
         pkh)
     Data_encoding.(
-      obj1 (req "consensus_pkh" Signature.Public_key_hash.encoding))
-    (function Invalid_consensus_key_update_active pkh -> Some pkh | _ -> None)
-    (fun pkh -> Invalid_consensus_key_update_active pkh) ;
+      obj2
+        (req "consensus_pkh" Signature.Public_key_hash.encoding)
+        (req "kind" Operation_repr.consensus_key_kind_encoding))
+    (function Invalid_consensus_key_update_active c -> Some c | _ -> None)
+    (fun c -> Invalid_consensus_key_update_active c) ;
   register_error_kind
     `Permanent
     ~id:"delegate.consensus_key.tz4"
     ~title:"Consensus key cannot be a tz4"
     ~description:"Consensus key cannot be a tz4 (BLS public key)."
-    ~pp:(fun ppf pk ->
+    ~pp:(fun ppf (pk, kind) ->
       Format.fprintf
         ppf
-        "The consensus key %a is forbidden as it is a BLS public key."
+        "The %a key %a is forbidden as it is a BLS public key."
+        Operation_repr.pp_consensus_key_kind
+        kind
         Bls.Public_key_hash.pp
         (Bls.Public_key.hash pk))
-    Data_encoding.(obj1 (req "delegate_pk" Bls.Public_key.encoding))
-    (function Invalid_consensus_key_update_tz4 pk -> Some pk | _ -> None)
-    (fun pk -> Invalid_consensus_key_update_tz4 pk) ;
+    Data_encoding.(
+      obj2
+        (req "delegate_pk" Bls.Public_key.encoding)
+        (req "kind" Operation_repr.consensus_key_kind_encoding))
+    (function Invalid_consensus_key_update_tz4 c -> Some c | _ -> None)
+    (fun c -> Invalid_consensus_key_update_tz4 c) ;
   register_error_kind
     `Permanent
     ~id:"delegate.consensus_key.another_delegate"
     ~title:"Consensus key cannot be another delegate"
     ~description:"Consensus key cannot be another delegate"
-    ~pp:(fun ppf pkh ->
+    ~pp:(fun ppf (pkh, kind) ->
       Format.fprintf
         ppf
-        "The key %a cannot be registered as a consensus key, as it is the \
-         manager key of another registered delegate."
+        "The key %a cannot be registered as a %a key, as it is the manager key \
+         of another registered delegate."
         Signature.Public_key_hash.pp
-        pkh)
+        pkh
+        Operation_repr.pp_consensus_key_kind
+        kind)
     Data_encoding.(
-      obj1 (req "consensus_pkh" Signature.Public_key_hash.encoding))
+      obj2
+        (req "consensus_pkh" Signature.Public_key_hash.encoding)
+        (req "kind" Operation_repr.consensus_key_kind_encoding))
     (function
-      | Invalid_consensus_key_update_another_delegate pkh -> Some pkh
-      | _ -> None)
-    (fun pkh -> Invalid_consensus_key_update_another_delegate pkh)
+      | Invalid_consensus_key_update_another_delegate c -> Some c | _ -> None)
+    (fun c -> Invalid_consensus_key_update_another_delegate c)
 
 type 'a typed_kind =
   | Consensus : Signature.public_key typed_kind
@@ -161,25 +176,25 @@ let pp ppf {delegate; consensus_pkh} =
    companion) that will be active at cycle `current + consensus_rights_delay + 1`.
 *)
 
-let check_unused ctxt pkh =
+let check_unused ctxt kind pkh =
   let open Lwt_result_syntax in
   let*! is_active = Storage.Consensus_keys.mem ctxt pkh in
-  fail_when is_active (Invalid_consensus_key_update_active pkh)
+  fail_when is_active (Invalid_consensus_key_update_active (pkh, kind))
 
-let check_not_tz4 : Signature.Public_key.t -> unit tzresult =
+let check_not_tz4 kind : Signature.Public_key.t -> unit tzresult =
   let open Result_syntax in
   function
-  | Bls pk -> tzfail (Invalid_consensus_key_update_tz4 pk)
+  | Bls pk -> tzfail (Invalid_consensus_key_update_tz4 (pk, kind))
   | Ed25519 _ | Secp256k1 _ | P256 _ -> return_unit
 
-let check_is_not_another_delegate ctxt pkh delegate =
+let check_is_not_another_delegate ctxt kind pkh delegate =
   let open Lwt_result_syntax in
   if Signature.Public_key_hash.equal pkh delegate then return_unit
   else
     let*! is_another_delegate = Storage.Delegates.mem ctxt pkh in
     fail_when
       is_another_delegate
-      (Invalid_consensus_key_update_another_delegate pkh)
+      (Invalid_consensus_key_update_another_delegate (pkh, kind))
 
 let set_unused = Storage.Consensus_keys.remove
 
@@ -188,7 +203,7 @@ let set_used = Storage.Consensus_keys.add
 let init ctxt delegate pk =
   let open Lwt_result_syntax in
   let pkh = Signature.Public_key.hash pk in
-  let* () = check_unused ctxt pkh in
+  let* () = check_unused ctxt Consensus pkh in
   let*! ctxt = set_used ctxt pkh in
   Storage.Contract.Consensus_key.init ctxt (Contract_repr.Implicit delegate) pk
 
@@ -302,8 +317,8 @@ let register_update ctxt delegate pk =
       (Invalid_consensus_key_update_noop (first_active_cycle, Consensus))
   in
   let pkh = Signature.Public_key.hash pk in
-  let* () = check_unused ctxt pkh in
-  let* () = check_is_not_another_delegate ctxt pkh delegate in
+  let* () = check_unused ctxt Consensus pkh in
+  let* () = check_is_not_another_delegate ctxt Consensus pkh delegate in
   let*! ctxt = set_used ctxt pkh in
   let* {consensus_pkh = old_pkh; _} =
     active_pubkey_for_cycle ctxt delegate update_cycle
@@ -338,8 +353,8 @@ let register_update_companion ctxt delegate pk =
   let pkh : Signature.public_key_hash =
     Signature.Bls (Bls.Public_key.hash pk)
   in
-  let* () = check_unused ctxt pkh in
-  let* () = check_is_not_another_delegate ctxt pkh delegate in
+  let* () = check_unused ctxt Companion pkh in
+  let* () = check_is_not_another_delegate ctxt Companion pkh delegate in
   let*! ctxt = set_used ctxt pkh in
   let* {companion_pkh = old_pkh; _} =
     active_pubkey_for_cycle ctxt delegate update_cycle
diff --git a/src/proto_alpha/lib_protocol/delegate_consensus_key.mli b/src/proto_alpha/lib_protocol/delegate_consensus_key.mli
index bbd0d14954547254822d53efb586fdeac18d31bc..18b7addb9fbbdbb88e10581c0e7ecdaf831b21a2 100644
--- a/src/proto_alpha/lib_protocol/delegate_consensus_key.mli
+++ b/src/proto_alpha/lib_protocol/delegate_consensus_key.mli
@@ -34,9 +34,12 @@
 type error +=
   | Invalid_consensus_key_update_noop of
       (Cycle_repr.t * Operation_repr.consensus_key_kind)
-  | Invalid_consensus_key_update_active of Signature.Public_key_hash.t
-  | Invalid_consensus_key_update_tz4 of Bls.Public_key.t
-  | Invalid_consensus_key_update_another_delegate of Signature.Public_key_hash.t
+  | Invalid_consensus_key_update_active of
+      (Signature.Public_key_hash.t * Operation_repr.consensus_key_kind)
+  | Invalid_consensus_key_update_tz4 of
+      (Bls.Public_key.t * Operation_repr.consensus_key_kind)
+  | Invalid_consensus_key_update_another_delegate of
+      (Signature.Public_key_hash.t * Operation_repr.consensus_key_kind)
 
 (** The public key of a consensus key and the associated delegate. *)
 type pk = Raw_context.consensus_pk = {
@@ -62,7 +65,8 @@ val pp : Format.formatter -> t -> unit
 val pkh : pk -> t
 
 (** [check_not_tz4 pk] checks that [pk] is not a BLS address. *)
-val check_not_tz4 : Signature.public_key -> unit tzresult
+val check_not_tz4 :
+  Operation_repr.consensus_key_kind -> Signature.public_key -> unit tzresult
 
 (** Initialize the consensus key when registering a delegate. *)
 val init :
diff --git a/src/proto_alpha/lib_protocol/test/integration/consensus/test_companion_key.ml b/src/proto_alpha/lib_protocol/test/integration/consensus/test_companion_key.ml
index 49ef87123c8cce3532b887eaf812676a30fade4a..67f207d4600ac61f012cfa6c0ec0928efbfe622f 100644
--- a/src/proto_alpha/lib_protocol/test/integration/consensus/test_companion_key.ml
+++ b/src/proto_alpha/lib_protocol/test/integration/consensus/test_companion_key.ml
@@ -68,23 +68,24 @@ type key_kind = Protocol.Operation_repr.consensus_key_kind =
   | Consensus
   | Companion
 
-let check_error_invalid_consensus_key_update_active ~loc ~pkh errs =
+let check_error_invalid_consensus_key_update_active ~loc ~pkh ~kind errs =
   Assert.expect_error ~loc errs (function
       | [
           Protocol.Delegate_consensus_key.Invalid_consensus_key_update_active
-            err_pkh;
+            (err_pkh, err_kind);
         ]
-        when Signature.Public_key_hash.equal pkh err_pkh ->
+        when Signature.Public_key_hash.equal pkh err_pkh && kind = err_kind ->
           true
       | _ -> false)
 
-let check_error_invalid_consensus_key_update_another_delegate ~loc ~pkh errs =
+let check_error_invalid_consensus_key_update_another_delegate ~loc ~pkh ~kind
+    errs =
   Assert.expect_error ~loc errs (function
       | [
           Protocol.Delegate_consensus_key
-          .Invalid_consensus_key_update_another_delegate err_pkh;
+          .Invalid_consensus_key_update_another_delegate (err_pkh, err_kind);
         ]
-        when Signature.Public_key_hash.equal pkh err_pkh ->
+        when Signature.Public_key_hash.equal pkh err_pkh && kind = err_kind ->
           true
       | _ -> false)
 
@@ -336,13 +337,21 @@ let test_register_other_accounts_as_ck =
         ~loc:__LOC__
         ~expected_error:(fun (_block, state) err ->
           let pkh = (State.find_account "victim_1" state).pkh in
-          check_error_invalid_consensus_key_update_active ~loc:__LOC__ ~pkh err)
+          check_error_invalid_consensus_key_update_active
+            ~loc:__LOC__
+            ~pkh
+            ~kind:Consensus
+            err)
         (update_consensus_key ~ck_name:"victim_1" "delegate")
   --> assert_failure
         ~loc:__LOC__
         ~expected_error:(fun (_block, state) err ->
           let pkh = (State.find_account "victim_2" state).pkh in
-          check_error_invalid_consensus_key_update_active ~loc:__LOC__ ~pkh err)
+          check_error_invalid_consensus_key_update_active
+            ~loc:__LOC__
+            ~pkh
+            ~kind:Companion
+            err)
         (update_companion_key ~ck_name:"victim_2" "delegate")
   (* ... So we give them other consensus keys.
      Note that the algo is not defined, so it is chosen at random. *)
@@ -358,6 +367,7 @@ let test_register_other_accounts_as_ck =
           check_error_invalid_consensus_key_update_another_delegate
             ~loc:__LOC__
             ~pkh
+            ~kind:Consensus
             err)
         (update_consensus_key ~ck_name:"victim_1" "delegate")
   --> assert_failure
@@ -367,6 +377,7 @@ let test_register_other_accounts_as_ck =
           check_error_invalid_consensus_key_update_another_delegate
             ~loc:__LOC__
             ~pkh
+            ~kind:Companion
             err)
         (update_companion_key ~ck_name:"victim_2" "delegate")
 
@@ -390,7 +401,11 @@ let test_self_register_as_companion =
         ~loc:__LOC__
         ~expected_error:(fun (_block, state) err ->
           let pkh = (State.find_account delegate state).pkh in
-          check_error_invalid_consensus_key_update_active ~loc:__LOC__ ~pkh err)
+          check_error_invalid_consensus_key_update_active
+            ~loc:__LOC__
+            ~pkh
+            ~kind:Companion
+            err)
         (update_companion_key ~ck_name:delegate delegate)
   (* We can change that *)
   --> add_account "consensus_key"
@@ -447,7 +462,11 @@ let test_self_register_as_companion =
         ~loc:__LOC__
         ~expected_error:(fun (_block, state) err ->
           let pkh = (State.find_account delegate state).pkh in
-          check_error_invalid_consensus_key_update_active ~loc:__LOC__ ~pkh err)
+          check_error_invalid_consensus_key_update_active
+            ~loc:__LOC__
+            ~pkh
+            ~kind:Consensus
+            err)
         (update_consensus_key ~ck_name:delegate delegate)
   --> add_account ~algo:Bls "companion_key"
   --> update_companion_key ~ck_name:"companion_key" delegate
@@ -538,6 +557,7 @@ let test_register_same_key_multiple_times =
           check_error_invalid_consensus_key_update_active
             ~loc:__LOC__
             ~pkh:ck.pkh
+            ~kind:Consensus
             err)
       (update_consensus_key ~ck_name:ck delegate)
     --> assert_failure
@@ -566,6 +586,7 @@ let test_register_same_key_multiple_times =
               check_error_invalid_consensus_key_update_active
                 ~loc:__LOC__
                 ~pkh:ck.pkh
+                ~kind:Companion
                 err)
           (update_companion_key ~ck_name:ck delegate)
   in
@@ -839,11 +860,12 @@ let test_forbidden_tz4 =
               Assert.expect_error ~loc:__LOC__ err (function
                   | [
                       Protocol.Delegate_consensus_key
-                      .Invalid_consensus_key_update_tz4 err_ck_bls_pk;
+                      .Invalid_consensus_key_update_tz4 (err_ck_bls_pk, err_kind);
                     ] ->
-                      Signature.Public_key.equal
-                        (Bls err_ck_bls_pk)
-                        ck_account.pk
+                      kind = err_kind
+                      && Signature.Public_key.equal
+                           (Bls err_ck_bls_pk)
+                           ck_account.pk
                   | _ -> false))
             (update_key ~kind ~ck_name:"ck" "delegate"))
         [("update consensus", Consensus); ("update companion", Companion)]
@@ -920,6 +942,7 @@ let test_fail_already_registered =
               check_error_invalid_consensus_key_update_active
                 ~loc:__LOC__
                 ~pkh
+                ~kind
                 err)
             (update_key ~kind ~ck_name:"ck" delegate))
         [("update consensus", Consensus); ("update companion", Companion)]
diff --git a/src/proto_alpha/lib_protocol/test/integration/consensus/test_consensus_key.ml b/src/proto_alpha/lib_protocol/test/integration/consensus/test_consensus_key.ml
index e7401d26766fcaa9c7a36ac3eed405bdfb962b73..3d06ba09e26083ae8209d4f21164dd853001771e 100644
--- a/src/proto_alpha/lib_protocol/test/integration/consensus/test_consensus_key.ml
+++ b/src/proto_alpha/lib_protocol/test/integration/consensus/test_consensus_key.ml
@@ -294,9 +294,9 @@ let test_tz4_consensus_key ~allow_tz4_delegate_enable () =
     let expect_failure = function
       | [
           Environment.Ecoproto_error
-            (Delegate_consensus_key.Invalid_consensus_key_update_tz4 pk);
+            (Delegate_consensus_key.Invalid_consensus_key_update_tz4 (pk, kind));
         ]
-        when Signature.Bls.Public_key.(pk = tz4_pk) ->
+        when Signature.Bls.Public_key.(pk = tz4_pk) && kind = Consensus ->
           return_unit
       | err ->
           failwith
diff --git a/src/proto_alpha/lib_protocol/test/unit/test_consensus_key.ml b/src/proto_alpha/lib_protocol/test/unit/test_consensus_key.ml
index 3cbf14100f98a75d0f0781128c63d9382ab06f72..c7ae7d372c62ef3aa6e87dda9462a35c2942bbc4 100644
--- a/src/proto_alpha/lib_protocol/test/unit/test_consensus_key.ml
+++ b/src/proto_alpha/lib_protocol/test/unit/test_consensus_key.ml
@@ -147,7 +147,8 @@ let test_consensus_key_storage () =
   let* () =
     let*! err = Consensus_key.register_update ctxt del1.pkh del2.pk in
     Assert.proto_error ~loc:__LOC__ err (function
-        | Delegate_consensus_key.Invalid_consensus_key_update_active pkh
+        | Delegate_consensus_key.Invalid_consensus_key_update_active
+            (pkh, Consensus)
           when Signature.Public_key_hash.equal del2.pkh pkh ->
             true
         | _ -> false)
@@ -163,7 +164,8 @@ let test_consensus_key_storage () =
   let* () =
     let*! err = Consensus_key.register_update ctxt del2.pkh a1.pk in
     Assert.proto_error ~loc:__LOC__ err (function
-        | Delegate_consensus_key.Invalid_consensus_key_update_active pkh
+        | Delegate_consensus_key.Invalid_consensus_key_update_active
+            (pkh, Consensus)
           when Signature.Public_key_hash.equal a1.pkh pkh ->
             true
         | _ -> false)
@@ -172,7 +174,7 @@ let test_consensus_key_storage () =
     let*! err = Consensus_key.register_update ctxt del2.pkh del1.pk in
     Assert.proto_error ~loc:__LOC__ err (function
         | Delegate_consensus_key.Invalid_consensus_key_update_another_delegate
-            pkh
+            (pkh, Consensus)
           when Signature.Public_key_hash.equal del1.pkh pkh ->
             true
         | _ -> false)
diff --git a/src/proto_alpha/lib_protocol/validate.ml b/src/proto_alpha/lib_protocol/validate.ml
index ad8b5fba90fd453dba3009d6fa4589307ad53913..14014e0eb697b5efeae6aceee985d836a8d1c74a 100644
--- a/src/proto_alpha/lib_protocol/validate.ml
+++ b/src/proto_alpha/lib_protocol/validate.ml
@@ -2915,7 +2915,7 @@ module Manager = struct
                {kind = Consensus; source; public_key})
       | (Ed25519 _ | Secp256k1 _ | P256 _), None, Consensus -> return_unit
     else
-      let* () = Delegate.Consensus_key.check_not_tz4 public_key in
+      let* () = Delegate.Consensus_key.check_not_tz4 kind public_key in
       match kind with
       | Companion ->
           result_error