diff -purN a/bc/execute.c b/bc/execute.c
--- a/bc/execute.c	2006-09-04 21:39:31.000000000 -0500
+++ b/bc/execute.c	2016-09-09 12:57:14.756000000 -0500
@@ -133,7 +133,11 @@ execute ()
 	  gp = functions[pc.pc_func].f_label;
 	  l_gp  = label_num >> BC_LABEL_LOG;
 	  l_off = label_num % BC_LABEL_GROUP;
-	  while (l_gp-- > 0) gp = gp->l_next;
+	  while (gp && l_gp-- > 0) gp = gp->l_next;
+	  if (!gp) {
+	    rt_error ("No label group for label %d.", label_num);
+	    break;
+	  }
 	  pc.pc_addr = gp->l_adrs[l_off];
 	}
 	break;