diff --git a/app/assets/javascripts/discourse/lib/url.js.es6 b/app/assets/javascripts/discourse/lib/url.js.es6
index 0f3364ab32..94ffa45229 100644
--- a/app/assets/javascripts/discourse/lib/url.js.es6
+++ b/app/assets/javascripts/discourse/lib/url.js.es6
@@ -173,8 +173,9 @@ const DiscourseURL = Ember.Object.createWithMixins({
    * @method isInternal
    * @param {String} url
   **/
-  isInternal: function(url) {
+  isInternal(url) {
     if (url && url.length) {
+      if (url.indexOf('//') === 0) { url = "http:" + url; }
       if (url.indexOf('#') === 0) { return true; }
       if (url.indexOf('/') === 0) { return true; }
       if (url.indexOf(this.origin()) === 0) { return true; }
diff --git a/test/javascripts/lib/url-test.js.es6 b/test/javascripts/lib/url-test.js.es6
index 39941fd00e..7c420b58cc 100644
--- a/test/javascripts/lib/url-test.js.es6
+++ b/test/javascripts/lib/url-test.js.es6
@@ -7,8 +7,10 @@ test("isInternal with a HTTP url", function() {
 
   not(DiscourseURL.isInternal(null), "a blank URL is not internal");
   ok(DiscourseURL.isInternal("/test"), "relative URLs are internal");
+  ok(DiscourseURL.isInternal("//eviltrout.com"), "a url on the same host is internal (protocol-less)");
   ok(DiscourseURL.isInternal("http://eviltrout.com/tophat"), "a url on the same host is internal");
   ok(DiscourseURL.isInternal("https://eviltrout.com/moustache"), "a url on a HTTPS of the same host is internal");
+  not(DiscourseURL.isInternal("//twitter.com.com"), "a different host is not internal (protocol-less)");
   not(DiscourseURL.isInternal("http://twitter.com"), "a different host is not internal");
 });