diff --git a/src/index.html b/src/index.html
index 7c95044..be72874 100644
--- a/src/index.html
+++ b/src/index.html
@@ -267,7 +267,7 @@ file, You can obtain one at http://mozilla.org/MPL/2.0/.
 </div>
 
 <div class="modal fade" id="alertModal" tabindex="-1" aria-labelledby="alertModalLabel" aria-hidden="true">
-    <div class="modal-dialog modal-lg">
+    <div class="modal-dialog modal-dialog-centered modal-lg">
         <div class="modal-content">
             <div class="modal-body p-1"></div>
             <div class="modal-footer p-1">