diff --git a/lib/output.js b/lib/output.js
index 7cd3e3c0..5f2f46d1 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -812,7 +812,8 @@ function OutputStream(options) {
             if (len > 0) output.space();
             a.forEach(function(exp, i){
                 if (i) output.comma();
-                exp.print(output);
+                if (!(exp instanceof AST_Undefined))
+                    exp.print(output);
             });
             if (len > 0) output.space();
         });