diff --git a/bgpd/bgp_mplsvpn.c b/bgpd/bgp_mplsvpn.c
index 72ad089..c1f1fbb 100644
--- a/bgpd/bgp_mplsvpn.c
+++ b/bgpd/bgp_mplsvpn.c
@@ -233,9 +233,13 @@
   char *endptr;
   u_int32_t t;
 
-  l = strtoul (str, &endptr, 10);
+  if (*str == '-')
+    return 0;
   
-  if (*endptr == '\0' || l == ULONG_MAX || l > UINT32_MAX)
+  errno = 0;
+  l = strtoul (str, &endptr, 10);
+
+  if (*endptr != '\0' || errno || l > UINT32_MAX)
     return 0;
 
   t = (u_int32_t) l;