diff --git a/lib/javascript/gflib.js b/lib/javascript/gflib.js
index 6c5b77c66..15be18b3d 100644
--- a/lib/javascript/gflib.js
+++ b/lib/javascript/gflib.js
@@ -34,7 +34,7 @@ Fun.prototype.show = function (prec) {
 		var s = this.name;
 		var cs = this.args;
 		for (var i in cs) {
-			s += " " + cs[i].show(1);
+		        s += " " + (isUndefined(cs[i]) ? "undefined" : cs[i].show(1));
 		}
 		if (prec > 0 && cs.length > 0) {
 			s = "(" + s + ")" ;