diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 9bd28f1..2345944 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -254,8 +254,18 @@
     if (element_settings.dialog) {
       ajax.options.data.dialogOptions = element_settings.dialog;
     }
+
     if (element_settings.dialogType) {
-      ajax.options.url += '?dialogType=' + element_settings.dialogType;
+      var query_parameters_prefix = '';
+      // Ensure that we have a valid URL by adding ? when no query parameter is
+      // yet available, otherwise append using &.
+      if (ajax.options.url.indexOf('?') === -1) {
+        query_parameters_prefix = '?';
+      }
+      else {
+        query_parameters_prefix = '&';
+      }
+      ajax.options.url += query_parameters_prefix + 'dialogType=' + element_settings.dialogType;
     }
 
     // Bind the ajaxSubmit function to the element event.