diff --git a/lib/if.h b/lib/if.h
index 0f088a9..218f102 100644
--- a/lib/if.h
+++ b/lib/if.h
@@ -94,8 +94,8 @@
   int metric;
 
   /* Interface MTU. */
-  int mtu;    /* IPv4 MTU */
-  int mtu6;   /* IPv6 MTU - probably, but not neccessarily same as mtu */
+  unsigned int mtu;    /* IPv4 MTU */
+  unsigned int mtu6;   /* IPv6 MTU - probably, but not neccessarily same as mtu */
 
   /* Hardware address. */
 #ifdef HAVE_SOCKADDR_DL