diff --git a/lib/output.js b/lib/output.js
index a29e6969..d4c417a5 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -96,7 +96,7 @@ function OutputStream(options) {
               case '"': ++dq; return '"';
               case "'": ++sq; return "'";
               case "\0":
-                var next = s.charCodeAt(i + 1);
+                var next = str.charCodeAt(i + 1);
                 return 48 <= next && next <= 57 ? "\\x00" : "\\0";
             }
             return s;