diff --git a/lib/sys.js b/lib/sys.js
index 7aba932205b..28ec6ff740e 100644
--- a/lib/sys.js
+++ b/lib/sys.js
@@ -33,7 +33,12 @@ exports.inspect = function (obj, showHidden, depth) {
   var seen = [];
   function format(value, recurseTimes) {
     // Provide a hook for user-specified inspect functions.
-    if (value && typeof value.inspect === 'function') {
+    // Check that value is an object with an inspect function on it
+    if (value && typeof value.inspect === 'function' &&
+        // Filter out the sys module, it's inspect function is special
+        value !== exports &&
+        // Also filter out any prototype objects using the circular check.
+        !(value.constructor && value.constructor.prototype === value)) {
       return value.inspect(recurseTimes);
     }