diff --git a/src/if_mzsch.c b/src/if_mzsch.c
index 9eb524a..6a34ad3 100644
--- a/src/if_mzsch.c
+++ b/src/if_mzsch.c
@@ -1249,7 +1249,7 @@
 
     expr = SCHEME_STR_VAL(GUARANTEE_STRING(prim->name, 0));
 
-    str = (char *)eval_to_string((char_u *)expr, NULL);
+    str = (char *)eval_to_string((char_u *)expr, NULL, TRUE);
 
     if (str == NULL)
 	raise_vim_exn(_("invalid expression"));