diff --git a/index.js b/index.js
index 3a079d4..2ee0081 100644
--- a/index.js
+++ b/index.js
@@ -245,7 +245,7 @@ function arrObjKeys (obj, inspect) {
 // Returns the object's constructor name or null if it is a plain object
 // or doesn't have a prototype.
 function getTypeString(o) {
-    if (Object.prototype.toString(o) !== '[object Object]') return null;
+    if (Object.prototype.toString.call(o) !== '[object Object]') return null;
     var prototype = getPrototype(o);
     if (!prototype) return null;