diff --git a/src/index.jst b/src/index.jst
index 5d4ee2f..cb0035d 100644
--- a/src/index.jst
+++ b/src/index.jst
@@ -48,8 +48,8 @@ module.exports = function equal(a, b) {
 {{?}}
 
     if (a.constructor === RegExp) return a.source === b.source && a.flags === b.flags;
-    if (a.valueOf !== Object.prototype.valueOf) return a.valueOf() === b.valueOf();
-    if (a.toString !== Object.prototype.toString) return a.toString() === b.toString();
+    if (a.valueOf !== Object.prototype.valueOf && a.valueOf) return a.valueOf() === b.valueOf();
+    if (a.toString !== Object.prototype.toString && a.toString) return a.toString() === b.toString();
 
     keys = Object.keys(a);
     length = keys.length;