diff --git a/interpreter/exec/simd.ml b/interpreter/exec/simd.ml
index b61b44794..5a2b83de3 100644
--- a/interpreter/exec/simd.ml
+++ b/interpreter/exec/simd.ml
@@ -20,8 +20,6 @@ sig
   val to_string : t -> string
   val bytewidth : int
   val of_strings : shape -> string list -> t
-  val of_bits : string -> t
-  val to_bits : t -> string
 end
 
 module type S =
@@ -35,14 +33,14 @@ sig
   val of_strings : shape -> string list -> t
 end
 
-module Make (Rep : RepType) : S with type bits = string =
+module Make (Rep : RepType) : S with type bits = Rep.t =
 struct
   type t = Rep.t
-  type bits = string
+  type bits = Rep.t
 
   let default = Rep.make Rep.bytewidth (chr 0)
   let to_string = Rep.to_string (* FIXME very very wrong *)
-  let of_bits = Rep.of_bits
-  let to_bits = Rep.to_bits
+  let of_bits x = x
+  let to_bits x = x
   let of_strings = Rep.of_strings
 end
diff --git a/interpreter/exec/v128.ml b/interpreter/exec/v128.ml
index 22a4bc121..33d6bd775 100644
--- a/interpreter/exec/v128.ml
+++ b/interpreter/exec/v128.ml
@@ -1,9 +1,8 @@
 include Simd.Make
   (struct
-    include Bytes
+    include String
     let bytewidth = 16
-    let of_bits = Bytes.of_string
-    let to_bits = Bytes.to_string
+    let to_string s = s
 
     let of_strings shape ss =
       if List.length ss <> Simd.lanes shape then raise (Invalid_argument "wrong length");
@@ -28,5 +27,5 @@ include Simd.Make
         List.iteri (fun i s -> set_int32_le b (i * 4) (F32.to_bits (F32.of_string s))) ss
       | Simd.F64x2 ->
         List.iteri (fun i s -> set_int64_le b (i * 8) (F64.to_bits (F64.of_string s))) ss);
-      b
+      Bytes.to_string b
   end)