diff --git a/src/proto_alpha/lib_benchmark/execution_context.ml b/src/proto_alpha/lib_benchmark/execution_context.ml
index 86fbdd7278e7e06ecbc7145e122df9f5e09f128a..a44d25fc872136505947f21c3d718837b35a21e4 100644
--- a/src/proto_alpha/lib_benchmark/execution_context.ml
+++ b/src/proto_alpha/lib_benchmark/execution_context.ml
@@ -58,9 +58,9 @@ let make ~rng_state =
   let level = Script_int.zero_n in
   let open Script_interpreter in
   (match context with
-  | `Mem_block (block, (bs1, bs2, _, _, _)) ->
+  | `Mem_block (block, (bs1, _, _, _, _)) ->
       let source = bs1 in
-      let payer = bs2 in
+      let payer = Contract_helpers.default_payer in
       let self = Contract_helpers.default_self in
       let step_constants =
         {
diff --git a/src/proto_alpha/lib_benchmarks_proto/interpreter_benchmarks.ml b/src/proto_alpha/lib_benchmarks_proto/interpreter_benchmarks.ml
index d372190f4510618057ec5fedf746585198488780..735f29f015b85cc054fd4142130c1f3ee49ba584 100644
--- a/src/proto_alpha/lib_benchmarks_proto/interpreter_benchmarks.ml
+++ b/src/proto_alpha/lib_benchmarks_proto/interpreter_benchmarks.ml
@@ -2978,11 +2978,10 @@ module Registration_section = struct
         ~cont_and_stack_sampler:(fun _cfg _rng_state ->
           let open Script_typed_ir in
           let open Alpha_context in
-          let zero = Contract.Implicit Signature.Public_key_hash.zero in
           let step_constants =
             {
-              source = zero;
-              payer = zero;
+              source = Contract.Implicit Signature.Public_key_hash.zero;
+              payer = Signature.Public_key_hash.zero;
               self = Contract_hash.zero;
               amount = Tez.zero;
               balance = Tez.zero;
diff --git a/src/proto_alpha/lib_client/client_proto_fa12.mli b/src/proto_alpha/lib_client/client_proto_fa12.mli
index 76800482921ca900650c3b0496ec9506b0b37a28..4817f9c68295ededefd170fb10fa918288f77be6 100644
--- a/src/proto_alpha/lib_client/client_proto_fa12.mli
+++ b/src/proto_alpha/lib_client/client_proto_fa12.mli
@@ -155,7 +155,7 @@ val run_view_action :
   ?source:Contract.t ->
   contract:Contract_hash.t ->
   action:action ->
-  ?payer:Contract.t ->
+  ?payer:Signature.public_key_hash ->
   ?gas:Gas.Arith.integral ->
   unparsing_mode:Script_ir_translator.unparsing_mode ->
   unit ->
diff --git a/src/proto_alpha/lib_client/client_proto_programs.ml b/src/proto_alpha/lib_client/client_proto_programs.ml
index e2e7d31e3803719f9df725ee37a1a49f05879354..728e9ec1baaa1e6c2ab1add4c4726f99ac79b753 100644
--- a/src/proto_alpha/lib_client/client_proto_programs.ml
+++ b/src/proto_alpha/lib_client/client_proto_programs.ml
@@ -126,7 +126,7 @@ type simulation_params = {
   now : Script_timestamp.t option;
   level : Script_int.n Script_int.num option;
   source : Contract.t option;
-  payer : Contract.t option;
+  payer : Signature.public_key_hash option;
   gas : Gas.Arith.integral option;
 }
 
diff --git a/src/proto_alpha/lib_client/client_proto_programs.mli b/src/proto_alpha/lib_client/client_proto_programs.mli
index dbe6e154e2ab9da1994cf9d051fca871ddd0cf6f..40d5acac1b4f50bc1df8a041a3a5c481f7c4a1dd 100644
--- a/src/proto_alpha/lib_client/client_proto_programs.mli
+++ b/src/proto_alpha/lib_client/client_proto_programs.mli
@@ -39,7 +39,7 @@ type simulation_params = {
   now : Script_timestamp.t option;
   level : Script_int.n Script_int.num option;
   source : Contract.t option;
-  payer : Contract.t option;
+  payer : Signature.public_key_hash option;
   gas : Gas.Arith.integral option;
 }
 
diff --git a/src/proto_alpha/lib_client_commands/client_proto_fa12_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_fa12_commands.ml
index 3c276d20c0df7a949d323a1681644a232f5657cb..e647c63f2f4e49b15db4a4fc6f698b50ae4ebabb 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_fa12_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_fa12_commands.ml
@@ -66,8 +66,8 @@ let as_arg =
     ()
 
 let payer_arg =
-  Client_proto_contracts.ContractAlias.destination_arg
-    ~name:"payer"
+  Client_keys.Public_key_hash.source_arg
+    ~long:"payer"
     ~doc:"name of the payer (i.e. SOURCE) contract for the transaction"
     ()
 
diff --git a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
index d82c6be1bc6c8e9a372a055057149908d1129358..39241974d1204dc7ce436f088fe77d967ecf5f12 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
@@ -91,8 +91,8 @@ let commands () =
       ()
   in
   let payer_arg =
-    ContractAlias.destination_arg
-      ~name:"payer"
+    Client_keys.Public_key_hash.source_arg
+      ~long:"payer"
       ~doc:"name of the payer (i.e. SOURCE) contract for the transaction"
       ()
   in
diff --git a/src/proto_alpha/lib_plugin/RPC.ml b/src/proto_alpha/lib_plugin/RPC.ml
index daaac1c42cd298cc851ca6ebb11c76622cedfbf8..4ef926e5a978f9ab0d58d94f4707a46313ae590a 100644
--- a/src/proto_alpha/lib_plugin/RPC.ml
+++ b/src/proto_alpha/lib_plugin/RPC.ml
@@ -164,7 +164,7 @@ module Scripts = struct
            (opt "balance" Tez.encoding)
            (req "chain_id" Chain_id.encoding)
            (opt "source" Contract.encoding)
-           (opt "payer" Contract.encoding)
+           (opt "payer" Contract.implicit_encoding)
            (opt "self" Contract.originated_encoding)
            (dft "entrypoint" Entrypoint.simple_encoding Entrypoint.default))
         (obj4
@@ -217,7 +217,7 @@ module Scripts = struct
         (req "input" Script.expr_encoding)
         (req "chain_id" Chain_id.encoding)
         (opt "source" Contract.encoding)
-        (opt "payer" Contract.encoding)
+        (opt "payer" Contract.implicit_encoding)
         (opt "gas" Gas.Arith.z_integral_encoding)
         (req "unparsing_mode" unparsing_mode_encoding)
         (opt "now" Script_timestamp.encoding)
@@ -233,7 +233,7 @@ module Scripts = struct
            (dft "unlimited_gas" bool false)
            (req "chain_id" Chain_id.encoding)
            (opt "source" Contract.encoding)
-           (opt "payer" Contract.encoding)
+           (opt "payer" Contract.implicit_encoding)
            (opt "gas" Gas.Arith.z_integral_encoding)
            (req "unparsing_mode" unparsing_mode_encoding)
            (opt "now" Script_timestamp.encoding))
@@ -901,7 +901,7 @@ module Scripts = struct
   type run_code_config = {
     balance : Tez.t;
     self : Contract_hash.t;
-    payer : Contract.t;
+    payer : Signature.public_key_hash;
     source : Contract.t;
   }
 
@@ -928,6 +928,14 @@ module Scripts = struct
         balance
       >>=? fun (ctxt, _) -> return (ctxt, dummy_contract_hash)
     in
+    let source_and_payer ~src_opt ~pay_opt ~default_src =
+      match (src_opt, pay_opt) with
+      | None, None ->
+          (Contract.Originated default_src, Signature.Public_key_hash.zero)
+      | Some c, None -> (c, Signature.Public_key_hash.zero)
+      | None, Some c -> (Contract.Implicit c, c)
+      | Some src, Some pay -> (src, pay)
+    in
     let configure_contracts ctxt script balance ~src_opt ~pay_opt ~self_opt =
       (match self_opt with
       | None ->
@@ -942,12 +950,7 @@ module Scripts = struct
           >>=? fun bal -> return (ctxt, addr, bal))
       >>=? fun (ctxt, self, balance) ->
       let source, payer =
-        match (src_opt, pay_opt) with
-        | None, None ->
-            let self = Contract.Originated self in
-            (self, self)
-        | Some c, None | None, Some c -> (c, c)
-        | Some src, Some pay -> (src, pay)
+        source_and_payer ~src_opt ~pay_opt ~default_src:self
       in
       return (ctxt, {balance; self; source; payer})
     in
@@ -1134,8 +1137,8 @@ module Scripts = struct
           entrypoint,
           input,
           chain_id,
-          source,
-          payer,
+          src_opt,
+          pay_opt,
           gas,
           unparsing_mode,
           now,
@@ -1160,11 +1163,7 @@ module Scripts = struct
              Tez.zero
         >>=? fun (ctxt, viewer_contract) ->
         let source, payer =
-          match (source, payer) with
-          | Some source, Some payer -> (source, payer)
-          | Some source, None -> (source, source)
-          | None, Some payer -> (payer, payer)
-          | None, None -> (contract, contract)
+          source_and_payer ~src_opt ~pay_opt ~default_src:contract_hash
         in
         let gas =
           Option.value
@@ -1234,8 +1233,8 @@ module Scripts = struct
             input,
             unlimited_gas,
             chain_id,
-            source,
-            payer,
+            src_opt,
+            pay_opt,
             gas,
             unparsing_mode,
             now ),
@@ -1253,11 +1252,7 @@ module Scripts = struct
         >>=? fun (input_ty, output_ty) ->
         Contract.get_balance ctxt contract >>=? fun balance ->
         let source, payer =
-          match (source, payer) with
-          | Some source, Some payer -> (source, payer)
-          | Some source, None -> (source, source)
-          | None, Some payer -> (payer, payer)
-          | None, None -> (contract, contract)
+          source_and_payer ~src_opt ~pay_opt ~default_src:contract_hash
         in
         let now =
           match now with None -> Script_timestamp.now ctxt | Some t -> t
diff --git a/src/proto_alpha/lib_protocol/alpha_context.mli b/src/proto_alpha/lib_protocol/alpha_context.mli
index ab984536b588abd62ca706fdaeca89db5fcfebcf..ef33c165a54e054d83553d0520050dc059c475e4 100644
--- a/src/proto_alpha/lib_protocol/alpha_context.mli
+++ b/src/proto_alpha/lib_protocol/alpha_context.mli
@@ -1633,6 +1633,8 @@ module Contract : sig
 
   include BASIC_DATA with type t := t
 
+  val implicit_encoding : Signature.public_key_hash Data_encoding.t
+
   val originated_encoding : Contract_hash.t Data_encoding.t
 
   val in_memory_size : t -> Cache_memory_helpers.sint
diff --git a/src/proto_alpha/lib_protocol/apply.ml b/src/proto_alpha/lib_protocol/apply.ml
index 89eb16234ab45b222f7e3210388f66801bcc6475..3f9accf971246dc31b5cf1516b998d69fb6907f8 100644
--- a/src/proto_alpha/lib_protocol/apply.ml
+++ b/src/proto_alpha/lib_protocol/apply.ml
@@ -837,7 +837,7 @@ let apply_transaction_to_smart_contract ~ctxt ~source ~contract_hash ~amount
         let open Script_interpreter in
         {
           source;
-          payer = Contract.Implicit payer;
+          payer;
           self = contract_hash;
           amount;
           chain_id;
@@ -3098,7 +3098,7 @@ let apply_liquidity_baking_subsidy ctxt ~toggle_vote =
                 entrypoint. *)
              {
                source = liquidity_baking_cpmm_contract;
-               payer = liquidity_baking_cpmm_contract;
+               payer = Signature.Public_key_hash.zero;
                self = liquidity_baking_cpmm_contract_hash;
                amount = liquidity_baking_subsidy;
                balance;
diff --git a/src/proto_alpha/lib_protocol/script_interpreter.ml b/src/proto_alpha/lib_protocol/script_interpreter.ml
index 0432fb11ef57b5ef64375da4393e693450546d98..9178b58ed9538b7cdc1ca8237103a84ad4536a04 100644
--- a/src/proto_alpha/lib_protocol/script_interpreter.ml
+++ b/src/proto_alpha/lib_protocol/script_interpreter.ml
@@ -91,7 +91,7 @@ module S = Saturation_repr
 
 type step_constants = Script_typed_ir.step_constants = {
   source : Contract.t;
-  payer : Contract.t;
+  payer : Signature.public_key_hash;
   self : Contract_hash.t;
   amount : Tez.t;
   balance : Tez.t;
@@ -1208,7 +1208,7 @@ and step : type a s b t r f. (a, s, b, t, r, f) step_type =
           let hash = Raw_hashes.sha512 bytes in
           (step [@ocaml.tailcall]) g gas k ks hash stack
       | ISource (_, k) ->
-          let destination : Destination.t = Contract sc.payer in
+          let destination : Destination.t = Contract (Implicit sc.payer) in
           let res = {destination; entrypoint = Entrypoint.default} in
           (step [@ocaml.tailcall]) g gas k ks res (accu, stack)
       | ISender (_, k) ->
diff --git a/src/proto_alpha/lib_protocol/script_interpreter.mli b/src/proto_alpha/lib_protocol/script_interpreter.mli
index 797a02f17f824d551ded98d018a0e0f2bf4a0f75..bcca460b020ff01665b55914e43a266cd0a43a44 100644
--- a/src/proto_alpha/lib_protocol/script_interpreter.mli
+++ b/src/proto_alpha/lib_protocol/script_interpreter.mli
@@ -62,7 +62,7 @@ type execution_result = {
 
 type step_constants = Script_typed_ir.step_constants = {
   source : Contract.t;
-  payer : Contract.t;
+  payer : Signature.public_key_hash;
   self : Contract_hash.t;
   amount : Tez.t;
   balance : Tez.t;
diff --git a/src/proto_alpha/lib_protocol/script_typed_ir.ml b/src/proto_alpha/lib_protocol/script_typed_ir.ml
index 60514a7975b4373920e4445072a319aecf99879d..e47943604c640e74284d6bd17af94cb3dce271ee 100644
--- a/src/proto_alpha/lib_protocol/script_typed_ir.ml
+++ b/src/proto_alpha/lib_protocol/script_typed_ir.ml
@@ -41,7 +41,7 @@ open Dependent_bool
 type step_constants = {
   source : Contract.t;
       (** The address calling this contract, as returned by SENDER. *)
-  payer : Contract.t;
+  payer : Signature.public_key_hash;
       (** The address of the implicit account that initiated the chain of contract calls, as returned by SOURCE. *)
   self : Contract_hash.t;
       (** The address of the contract being executed, as returned by SELF and SELF_ADDRESS.
diff --git a/src/proto_alpha/lib_protocol/script_typed_ir.mli b/src/proto_alpha/lib_protocol/script_typed_ir.mli
index 010b1ab6f20abd2681836ab42021857aac8121da..0af068f38163302b9085be70d12101c17f00d0df 100644
--- a/src/proto_alpha/lib_protocol/script_typed_ir.mli
+++ b/src/proto_alpha/lib_protocol/script_typed_ir.mli
@@ -31,7 +31,7 @@ open Dependent_bool
 
 type step_constants = {
   source : Contract.t;
-  payer : Contract.t;
+  payer : Signature.public_key_hash;
   self : Contract_hash.t;
   amount : Tez.t;
   balance : Tez.t;
diff --git a/src/proto_alpha/lib_protocol/test/helpers/contract_helpers.ml b/src/proto_alpha/lib_protocol/test/helpers/contract_helpers.ml
index 4053b8f6b9b59ddc9c504e196e044583d720c52b..ec4d76ef14250674f6e1508eb787a40ac6a67247 100644
--- a/src/proto_alpha/lib_protocol/test/helpers/contract_helpers.ml
+++ b/src/proto_alpha/lib_protocol/test/helpers/contract_helpers.ml
@@ -69,13 +69,15 @@ let fake_KT1 =
 
 let default_self = fake_KT1
 
-let default_source = Contract.Implicit Signature.Public_key_hash.zero
+let default_payer = Signature.Public_key_hash.zero
+
+let default_source = Contract.Implicit default_payer
 
 let default_step_constants =
   Script_interpreter.
     {
       source = default_source;
-      payer = default_source;
+      payer = default_payer;
       self = default_self;
       amount = Tez.zero;
       balance = Tez.zero;
diff --git a/tezt/tests/run_script.ml b/tezt/tests/run_script.ml
index 8b8ae4273b10254ef632013241f56a0f9c02d133..807b6fe5348e1aae89dd4aabae6c2ea572954f59 100644
--- a/tezt/tests/run_script.ml
+++ b/tezt/tests/run_script.ml
@@ -148,13 +148,21 @@ let test_source_and_sender ~protocol () =
   let* bootstrap1 = Client.show_address ~alias:"bootstrap1" client in
   let* bootstrap2 = Client.show_address ~alias:"bootstrap2" client in
 
-  (* When --payer is absent, --source sets *both* SENDER and SOURCE. *)
+  (* When --payer is absent, --source sets:
+     - *both* SENDER and SOURCE (until Kathmandu);
+     - SENDER, but SOURCE is the zero address (since L). *)
+  let expected_source =
+    match protocol with
+    | Ithaca | Jakarta | Kathmandu ->
+        Format.sprintf "%S" bootstrap1.public_key_hash
+    | Alpha -> "0x00000000000000000000000000000000000000000000"
+  in
   let* _storage =
     Client.run_script
       ~source:"bootstrap1"
       ~prg:check_source
       ~storage:"Unit"
-      ~input:(Format.sprintf "%S" bootstrap1.public_key_hash)
+      ~input:expected_source
       client
   in
   let* _storage =