diff --git a/lib/ip.js b/lib/ip.js
index 4b2adb5..0df8b8e 100644
--- a/lib/ip.js
+++ b/lib/ip.js
@@ -100,7 +100,7 @@ function _normalizeFamily(family) {
   if (family === 6) {
     return 'ipv6';
   }
-  return family ? family.toLowerCase() : 'ipv4';
+  return family ? family.toString().toLowerCase() : 'ipv4';
 }
 
 ip.fromPrefixLen = function (prefixlen, family) {