diff --git a/util/mcgg/scan.l b/util/mcgg/scan.l
index 447a7d395..9d06d5b38 100644
--- a/util/mcgg/scan.l
+++ b/util/mcgg/scan.l
@@ -30,6 +30,7 @@ static int braces = 0;
                                 yylval.string = strdup(yytext);
                                 return QFRAGMENT;
                             }
+<QSTRING>.                  yyerror("bad string input '%s' at line %d\n", yytext, yylineno);
 
 <INITIAL>"/*"               BEGIN(COMMENT);
 <COMMENT>"*/"               BEGIN(INITIAL);