diff --git a/lib/output.js b/lib/output.js
index 42b3aad9..08b49d64 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -1028,7 +1028,7 @@ function OutputStream(options) {
     DEFPRINT(AST_ObjectKeyVal, function(self, output){
         var key = self.key;
         if (output.option("quote_keys")) {
-            output.print_string(key);
+            output.print_string(key + "");
         } else if ((typeof key == "number"
                     || !output.option("beautify")
                     && +key + "" == key)