diff --git a/source/qb64.bas b/source/qb64.bas
index ca9412b65..aa533284b 100644
--- a/source/qb64.bas
+++ b/source/qb64.bas
@@ -17525,6 +17525,11 @@ FUNCTION evaluatetotyp$ (a2$, targettyp AS LONG)
         Give_Error "Illegal string-number conversion": EXIT FUNCTION
     END IF
 
+    '_MEM?
+    IF (sourcetyp AND (ISUDT + (1))) <> (targettyp AND (ISUDT + (1))) THEN
+        Give_Error "Illegal data type conversion (function returns _MEM type)": EXIT FUNCTION
+    END IF
+
     IF (sourcetyp AND ISSTRING) THEN
         evaluatetotyp$ = e$
         IF (sourcetyp AND ISREFERENCE) THEN