diff --git a/hosts/ocaml/lib/sx_vm.ml b/hosts/ocaml/lib/sx_vm.ml
index 6d0cc39..2d1635f 100644
--- a/hosts/ocaml/lib/sx_vm.ml
+++ b/hosts/ocaml/lib/sx_vm.ml
@@ -205,8 +205,13 @@ let rec run vm =
       (* Return — don't recurse, let caller continue *)
     | 51 (* OP_CLOSURE *) ->
       let idx = read_u16 frame in
+      if idx >= Array.length consts then
+        raise (Eval_error (Printf.sprintf "VM: CLOSURE idx %d >= consts %d" idx (Array.length consts)));
       let code_val = consts.(idx) in
       let code = code_from_value code_val in
+      Printf.eprintf "[vm-closure] idx=%d type=%s bc_len=%d consts=%d sp_before=%d\n%!"
+        idx (type_of code_val)
+        (Array.length code.bytecode) (Array.length code.constants) vm.sp;
       (* Read upvalue descriptors from bytecode *)
       let uv_count = match code_val with
         | Dict d -> (match Hashtbl.find_opt d "upvalue-count" with
@@ -246,10 +251,12 @@ let rec run vm =
       let args = List.init argc (fun _ -> pop vm) |> List.rev in
       let result =
         try
-          (* Check globals first (has env bindings like set-render-active!),
-             then fall back to registered primitives *)
-          let fn_val = try Hashtbl.find vm.globals name with Not_found ->
-            Sx_primitives.get_primitive name
+          (* Check primitives FIRST (native implementations of map/filter/etc.),
+             then globals (which may have ho_via_cek wrappers that route
+             through the CEK — these can't call VM closures). *)
+          let fn_val = try Sx_primitives.get_primitive name with _ ->
+            try Hashtbl.find vm.globals name with Not_found ->
+              raise (Eval_error ("VM: unknown primitive " ^ name))
           in
           (match fn_val with
            | NativeFn (_, fn) -> fn args
@@ -353,7 +360,13 @@ and call_closure cl args globals =
   List.iter (fun a -> push vm a) args;
   for _ = List.length args to cl.code.locals - 1 do push vm Nil done;
   vm.frames <- [frame];
-  run vm;
+  (try run vm
+   with e ->
+     Printf.eprintf "[vm-call-closure] FAIL in %s: %s (bc_len=%d args=%d sp=%d)\n%!"
+       (match cl.name with Some n -> n | None -> "?")
+       (Printexc.to_string e)
+       (Array.length cl.code.bytecode) (List.length args) vm.sp;
+     raise e);
   pop vm
 
 (** Execute a compiled module (top-level bytecode). *)