diff --git a/src/proto_alpha/lib_protocol/src/michelson_v1_gas.ml b/src/proto_alpha/lib_protocol/src/michelson_v1_gas.ml
index 61be2f502..6eaa1ec26 100644
--- a/src/proto_alpha/lib_protocol/src/michelson_v1_gas.ml
+++ b/src/proto_alpha/lib_protocol/src/michelson_v1_gas.ml
@@ -174,8 +174,12 @@ module Cost_of = struct
 
   let compare_res = step_cost 1
 
-  let unpack bytes = 10 *@ step_cost (MBytes.length bytes)
-  let pack bytes = alloc_bytes_cost (MBytes.length bytes)
+  let unpack_failed bytes =
+    (* We cannot instrument failed deserialization,
+       so we take worst case fees: a set of size 1 bytes values. *)
+    let len = MBytes.length bytes in
+    (len *@ alloc_mbytes_cost 1) +@
+    (len *@ (log2 len *@ (alloc_cost 3 +@ step_cost 1)))
 
   let address = step_cost 1
   let contract = Gas.read_bytes_cost Z.zero +@ step_cost 100
diff --git a/src/proto_alpha/lib_protocol/src/michelson_v1_gas.mli b/src/proto_alpha/lib_protocol/src/michelson_v1_gas.mli
index 882bd2977..1aa6dd5c9 100644
--- a/src/proto_alpha/lib_protocol/src/michelson_v1_gas.mli
+++ b/src/proto_alpha/lib_protocol/src/michelson_v1_gas.mli
@@ -64,8 +64,7 @@ module Cost_of : sig
   val exec : Gas.cost
   val push : Gas.cost
   val compare_res : Gas.cost
-  val pack : MBytes.t -> Gas.cost
-  val unpack : MBytes.t -> Gas.cost
+  val unpack_failed : MBytes.t -> Gas.cost
   val address : Gas.cost
   val contract : Gas.cost
   val transfer : Gas.cost
diff --git a/src/proto_alpha/lib_protocol/src/script_interpreter.ml b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
index 904dfd6a4..12e0da61a 100644
--- a/src/proto_alpha/lib_protocol/src/script_interpreter.ml
+++ b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
@@ -641,13 +641,18 @@ let rec interp
             Script_ir_translator.pack_data ctxt t value >>=? fun (bytes, ctxt) ->
             logged_return (Item (bytes, rest), ctxt)
         | Unpack t, Item (bytes, rest) ->
-            Lwt.return (Gas.consume ctxt (Interp_costs.pack bytes)) >>=? fun ctxt ->
-            begin match Data_encoding.Binary.of_bytes Script.expr_encoding bytes with
+            begin match Data_encoding.Binary.of_bytes Script.lazy_expr_encoding bytes with
               | None ->
+                  Lwt.return (Gas.consume ctxt (Interp_costs.unpack_failed bytes)) >>=? fun ctxt ->
                   logged_return (Item (None, rest), ctxt)
-              | Some expr ->
-                  parse_data ctxt t (Micheline.root expr) >>=? fun (value, ctxt) ->
-                  logged_return (Item (Some value, rest), ctxt)
+              | Some lexpr ->
+                  (Script.force_decode ctxt lexpr >>=? fun (expr, ctxt) ->
+                   parse_data ctxt t (Micheline.root expr)) >>= function
+                  | Ok (value, ctxt) ->
+                      logged_return (Item (Some value, rest), ctxt)
+                  | Error _ignored ->
+                      Lwt.return (Gas.consume ctxt (Interp_costs.unpack_failed bytes)) >>=? fun ctxt ->
+                      logged_return (Item (None, rest), ctxt)
             end
         (* protocol *)
         | Address, Item ((_, contract), rest) ->