Fix bug in expressInterestHelper: if interest.interestLifetime is null, use 4000.
diff --git a/js/NDN.js b/js/NDN.js
index edc4d84..9e76e4a 100644
--- a/js/NDN.js
+++ b/js/NDN.js
@@ -205,6 +205,7 @@
 	// Set interest timer
     var thisNDN = this;
 	if (closure != null) {
+        var timeoutMilliseconds = (interest.interestLifetime || 4000);
 		pitEntry.timerID = setTimeout(function() {
 			if (LOG > 3) console.log("Interest time out.");
 				
@@ -219,7 +220,7 @@
 				
 			// Raise closure callback
 			closure.upcall(Closure.UPCALL_INTEREST_TIMED_OUT, new UpcallInfo(thisNDN, interest, 0, null));
-		}, interest.interestLifetime);  // interestLifetime is in milliseconds.
+		}, timeoutMilliseconds);
 		//console.log(closure.timerID);
 	}