diff --git a/js/libtextsecure.js b/js/libtextsecure.js
index 9468c76424..aeccb62c3a 100644
--- a/js/libtextsecure.js
+++ b/js/libtextsecure.js
@@ -38790,6 +38790,9 @@ var TextSecureServer = (function() {
     var attachment_id_regex = RegExp( "^https:\/\/.*\/(\\d+)\?");
 
     function TextSecureServer(url, username, password) {
+        if (typeof url !== 'string') {
+            throw new Error('Invalid server url');
+        }
         this.url = url;
         this.username = username;
         this.password = password;
diff --git a/libtextsecure/api.js b/libtextsecure/api.js
index 8ea5806af9..7bec300ffd 100644
--- a/libtextsecure/api.js
+++ b/libtextsecure/api.js
@@ -73,6 +73,9 @@ var TextSecureServer = (function() {
     var attachment_id_regex = RegExp( "^https:\/\/.*\/(\\d+)\?");
 
     function TextSecureServer(url, username, password) {
+        if (typeof url !== 'string') {
+            throw new Error('Invalid server url');
+        }
         this.url = url;
         this.username = username;
         this.password = password;