diff --git a/compiler/parsing/ast_to_cexpScript.sml b/compiler/parsing/ast_to_cexpScript.sml
index c5938651..01af1448 100644
--- a/compiler/parsing/ast_to_cexpScript.sml
+++ b/compiler/parsing/ast_to_cexpScript.sml
@@ -388,10 +388,10 @@ Definition translate_type_def:
    else if s = "Bool" then do assert (tys = []); return $ PrimTy Bool; od
    else if s = "Integer" then do assert (tys = []); return $ PrimTy Integer od
    else if s = "String" then do assert (tys = []); return $ PrimTy String od
-   else if s = "M" then do assert (LENGTH tys = 1);
-                           t <- translate_type nm_map arg_map (HD tys);
-                           return $ M t;
-                        od
+   else if s = "IO" then do assert (LENGTH tys = 1);
+                            t <- translate_type nm_map arg_map (HD tys);
+                            return $ M t;
+                         od
    else if s = "Array" then do assert (LENGTH tys = 1);
                                t <- translate_type nm_map arg_map (HD tys);
                                return $ Array t;