diff --git a/lib/request_urls.js b/lib/request_urls.js
index 04383d1..ffd0900 100644
--- a/lib/request_urls.js
+++ b/lib/request_urls.js
@@ -37,13 +37,22 @@ function request_urls(config, urls, callback) {
   var delay = test_interval.getBackoff();
 
   var getOneUrl = function (){
-    if( urls.length === 0 ){
+    // check if all responses have been recieved and call the next step in
+    // processing via the `callback` function
+    if( Object.keys(responses).length === total_length ){
       clearInterval(intervalId);
-      return callback([]);
+      return callback( responses );
     }
 
     var url = urls.pop();
 
+    // if there are no more URLs to send requests for, we might
+    // be waiting for some to finish, so just return and the next call to
+    // this function from the ExponentialBackoff manager will try again
+    if (!url) {
+      return;
+    }
+
     const agent = (url.startsWith('https:')) ? httpsAgent : httpAgent;
 
     var requestOpts = {