diff --git a/lib/form/dndupload.js b/lib/form/dndupload.js
index 4be065040c7..f93f261c000 100644
--- a/lib/form/dndupload.js
+++ b/lib/form/dndupload.js
@@ -355,7 +355,10 @@ M.form_dndupload.init = function(Y, options) {
          * @return boolean true if event has files
          */
         has_files: function(e) {
-            var types = e._event.dataTransfer.types;
+            // In some browsers, dataTransfer.types may be null for a
+            // 'dragover' event, so ensure a valid Array is always
+            // inspected.
+            var types = e._event.dataTransfer.types || [];
             for (var i=0; i<types.length; i++) {
                 if (types[i] == 'Files') {
                     return true;