diff --git a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
index 3c7c9d315a9808335dc83db94ee0c17b739d934f..e63e7862b5d8d573359950e9b288217417eaeecd 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
@@ -24,6 +24,31 @@
 (*                                                                           *)
 (*****************************************************************************)
 
+type error += Sc_rollup_proof_check of string
+
+type error += Sc_rollup_invalid_serialized_inbox_proof
+
+let () =
+  register_error_kind
+    `Permanent
+    ~id:"Sc_rollup_proof_check"
+    ~title:"Invalid proof"
+    ~description:"An invalid proof has been submitted"
+    ~pp:(fun fmt msg -> Format.fprintf fmt "Invalid proof: %s" msg)
+    Data_encoding.(obj1 @@ req "reason" string)
+    (function Sc_rollup_proof_check msg -> Some msg | _ -> None)
+    (fun msg -> Sc_rollup_proof_check msg) ;
+
+  register_error_kind
+    `Permanent
+    ~id:"Sc_rollup_invalid_serialized_inbox_proof"
+    ~title:"Invalid serialized inbox proof"
+    ~description:"The serialized inbox proof can not be de-serialized"
+    ~pp:(fun fmt () -> Format.fprintf fmt "Invalid serialized inbox proof")
+    Data_encoding.unit
+    (function Sc_rollup_invalid_serialized_inbox_proof -> Some () | _ -> None)
+    (fun () -> Sc_rollup_invalid_serialized_inbox_proof)
+
 type t = {
   pvm_step : Sc_rollups.wrapped_proof;
   inbox : Sc_rollup_inbox_repr.serialized_proof option;
@@ -59,19 +84,6 @@ let cut_at_level level input =
   let input_level = Sc_rollup_PVM_sem.(input.inbox_level) in
   if Raw_level_repr.(level <= input_level) then None else Some input
 
-type error += Sc_rollup_proof_check of string
-
-let () =
-  register_error_kind
-    `Permanent
-    ~id:"Sc_rollup_proof_check"
-    ~title:"Invalid proof"
-    ~description:"An invalid proof has been submitted"
-    ~pp:(fun fmt msg -> Format.fprintf fmt "Invalid proof: %s" msg)
-    Data_encoding.(obj1 @@ req "reason" string)
-    (function Sc_rollup_proof_check msg -> Some msg | _ -> None)
-    (fun msg -> Sc_rollup_proof_check msg)
-
 let proof_error reason =
   let open Lwt_tzresult_syntax in
   fail (Sc_rollup_proof_check reason)
@@ -82,7 +94,7 @@ let check p reason =
 
 let check_inbox_proof snapshot serialized_inbox_proof (level, counter) =
   match Sc_rollup_inbox_repr.of_serialized_proof serialized_inbox_proof with
-  | None -> return None
+  | None -> fail Sc_rollup_invalid_serialized_inbox_proof
   | Some inbox_proof ->
       Sc_rollup_inbox_repr.verify_proof (level, counter) snapshot inbox_proof
 
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.mli b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.mli
index 81efa00c44362ba2a517b99a66a644d71dcf0d25..cf237cc62de1d56282641bdef8b6961f58c99325 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.mli
+++ b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.mli
@@ -64,6 +64,8 @@ type t = {
 
 type error += Sc_rollup_proof_check of string
 
+type error += Sc_rollup_invalid_serialized_inbox_proof
+
 val encoding : t Data_encoding.t
 
 val pp : Format.formatter -> t -> unit
diff --git a/src/proto_alpha/lib_protocol/test/unit/test_sc_rollup_game.ml b/src/proto_alpha/lib_protocol/test/unit/test_sc_rollup_game.ml
index 55a6005e4ac62e897b6c729c28cfd06723190b8f..00f50d5171a2b96ca4a65aa4541088138ace4289 100644
--- a/src/proto_alpha/lib_protocol/test/unit/test_sc_rollup_game.ml
+++ b/src/proto_alpha/lib_protocol/test/unit/test_sc_rollup_game.ml
@@ -306,6 +306,48 @@ let test_staker_injectivity () =
     res
     (( = ) (Sc_rollup_errors.Sc_rollup_staker_in_game (`Defender defender)))
 
+module Arith_pvm = Sc_rollup_helpers.Arith_pvm
+
+(** Test that sending a invalid serialized inbox proof to
+    {Sc_rollup_proof_repr.valid} is rejected. *)
+let test_invalid_serialized_inbox_proof () =
+  let open Lwt_result_syntax in
+  let open Alpha_context in
+  let* ctxt = Test_sc_rollup_inbox.create_context () in
+  let*! inbox =
+    Sc_rollup.Inbox.empty ctxt Sc_rollup.Address.zero Raw_level.root
+  in
+  let snapshot = Sc_rollup.Inbox.take_snapshot inbox in
+
+  let ctxt = Tezos_context_memory.make_empty_context () in
+  let*! state = Arith_pvm.initial_state ctxt in
+  (* We evaluate the boot sector, so the [input_requested] is a
+     [First_after]. *)
+  let*! state = Arith_pvm.eval state in
+  let*! proof = Arith_pvm.produce_proof ctxt None state in
+  let proof = WithExceptions.Result.get_ok ~loc:__LOC__ proof in
+  let wrapped_proof =
+    Sc_rollup.Arith_pvm_with_proof
+      (module struct
+        include Arith_pvm
+
+        let proof = proof
+      end)
+  in
+
+  (* We create an obviously invalid inbox *)
+  let inbox = Bytes.of_string "I am the big bad wolf" |> Obj.magic in
+  let proof = Sc_rollup.Proof.{pvm_step = wrapped_proof; inbox = Some inbox} in
+
+  let*! res =
+    T.lift
+    @@ Sc_rollup.Proof.valid snapshot Raw_level.root ~pvm_name:"arith" proof
+  in
+  Assert.proto_error
+    ~loc:__LOC__
+    res
+    (( = ) Sc_rollup_proof_repr.Sc_rollup_invalid_serialized_inbox_proof)
+
 let tests =
   [
     Tztest.tztest
@@ -320,4 +362,8 @@ let tests =
       "Staker can be in at most one game (injectivity)."
       `Quick
       test_staker_injectivity;
+    Tztest.tztest
+      "Invalid serialized inbox proof is rejected."
+      `Quick
+      test_invalid_serialized_inbox_proof;
   ]