diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index e9afd68df..4ec8e8693 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -13,9 +13,19 @@
 
 (rule
  (target version-dependent.wat)
- (deps version-dependent/pre-5.2.wat)
+ (deps version-dependent/post-5.2.wat)
+ (enabled_if
+  (and
+   (>= %{ocaml_version} 5.1.0)
+   (< %{ocaml_version} 5.2.0)))
+ (action
+  (copy %{deps} %{target})))
+
+(rule
+ (target version-dependent.wat)
+ (deps version-dependent/pre-5.1.wat)
  (enabled_if
-  (< %{ocaml_version} 5.2.0))
+  (< %{ocaml_version} 5.1.0))
  (action
   (copy %{deps} %{target})))
 
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 491d84385..46373b2bf 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -48,8 +48,8 @@
       (func $caml_find_custom_operations
          (param (ref $string)) (result (ref null $custom_operations))))
 
-   (global $caml_marshal_header_size (export "caml_marshal_header_size")
-      (mut i32) (i32.const 20))
+   (import "version-dependent" "caml_marshal_header_size"
+      (global $caml_marshal_header_size i32))
 
    (global $input_val_from_string (ref $string)
       (array.new_fixed $string 21
diff --git a/runtime/wasm/version-dependent/post-5.2.wat b/runtime/wasm/version-dependent/post-5.2.wat
index cd99832f1..b4183d2dc 100644
--- a/runtime/wasm/version-dependent/post-5.2.wat
+++ b/runtime/wasm/version-dependent/post-5.2.wat
@@ -54,4 +54,6 @@
             (ref.i31 (i32.const 0))
             (array.new_fixed $block 2 (ref.i31 (i32.const 0)) (local.get $res))))
       (ref.i31 (local.get $id)))
+
+   (global (export "caml_marshal_header_size") i32 (i32.const 16))
 )
diff --git a/runtime/wasm/version-dependent/pre-5.2.wat b/runtime/wasm/version-dependent/pre-5.1.wat
similarity index 96%
rename from runtime/wasm/version-dependent/pre-5.2.wat
rename to runtime/wasm/version-dependent/pre-5.1.wat
index 27c671a24..cc23b90ad 100644
--- a/runtime/wasm/version-dependent/pre-5.2.wat
+++ b/runtime/wasm/version-dependent/pre-5.1.wat
@@ -41,4 +41,6 @@
       (global.set $caml_domain_id (local.get $old))
       (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
       (ref.i31 (local.get $id)))
+
+   (global (export "caml_marshal_header_size") i32 (i32.const 20))
 )