diff --git a/lib/ipaddr.js b/lib/ipaddr.js
index 9d9b007..f516d1b 100644
--- a/lib/ipaddr.js
+++ b/lib/ipaddr.js
@@ -382,7 +382,7 @@
 
   ipaddr.IPv6.isValid = function(string) {
     var e;
-    if (string.indexOf(":") === -1) {
+    if (typeof string !== 'string' || string.indexOf(":") === -1) {
       return false;
     }
     try {