diff --git a/src/asm/globlex.c b/src/asm/globlex.c
index c26cdc4d..88ae4167 100644
--- a/src/asm/globlex.c
+++ b/src/asm/globlex.c
@@ -219,9 +219,15 @@ PutMacroArg(char *src, ULONG size)
 ULONG 
 PutUniqueArg(char *src, ULONG size)
 {
+	char *s;
+
 	src = src;
 	yyskipbytes(size);
-	yyunputstr(sym_FindMacroArg(-1));
+	if ((s = sym_FindMacroArg(-1)) != NULL) {
+		yyunputstr(s);
+	} else {
+		yyerror("Macro unique label string not defined");
+	}
 	return (0);
 }