diff --git a/lib/if.c b/lib/if.c
index ff5ec50..2e36eff 100644
--- a/lib/if.c
+++ b/lib/if.c
@@ -74,10 +74,14 @@
       return res;
 
     /* with identical name part, go to numeric part */
-
     p1 += l1;
     p2 += l1;
 
+    if (!*p1) 
+      return -1;
+    if (!*p2) 
+      return 1;
+
     x1 = strtol(p1, &p1, 10);
     x2 = strtol(p2, &p2, 10);