Index: regcomp.c
===================================================================
--- regcomp.c.orig
+++ regcomp.c
@@ -10742,7 +10742,7 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I
 
                 ret = reg2Lanode(pRExC_state, GOSUB, num, RExC_recurse_count);
                 if (!SIZE_ONLY) {
-		    if (num > (I32)RExC_rx->nparens) {
+		    if (num < 0 || num > (I32)RExC_rx->nparens) {
 			RExC_parse++;
 			vFAIL("Reference to nonexistent group");
 	            }