Index: src/layer_grp.c
===================================================================
--- src/layer_grp.c	(revision 10448)
+++ src/layer_grp.c	(revision 10449)
@@ -692,6 +692,8 @@
 	d = (gid1 < gid2) ? +1 : -1;
 	cnt = 0;
 	for(gid = gid1; gid != gid2; gid += d) {
+		if ((gid < 0) || (gid >= pcb->LayerGroups.len))
+			return -1;
 		if ((pcb->LayerGroups.grp[gid].type & mask) == mask)
 			cnt++;
 	}