--- ./regcomp.c.orig	2014-05-26 13:34:20.000000000 +0000
+++ ./regcomp.c	2014-06-02 14:39:25.188429940 +0000
@@ -9771,7 +9771,7 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I
 
                 ret = reganode(pRExC_state, GOSUB, num);
                 if (!SIZE_ONLY) {
-		    if (num > (I32)RExC_rx->nparens) {
+		    if (num < 0 || num > (I32)RExC_rx->nparens) {
 			RExC_parse++;
 			vFAIL("Reference to nonexistent group");
 	            }