diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -467,8 +467,8 @@
     $(this.element).addClass('progress-disabled').prop('disabled', true);
 
     // Insert progress indicator
-    var progressIndicatorMethod = 'setProgressIndicator' + this.progress.type.slice(0,1).toUpperCase() + this.progress.type.slice(1).toLowerCase();
-    if (this[progressIndicatorMethod] && this[progressIndicatorMethod].call) {
+    var progressIndicatorMethod = 'setProgressIndicator' + this.progress.type.slice(0, 1).toUpperCase() + this.progress.type.slice(1).toLowerCase();
+    if (progressIndicatorMethod in this && typeof this[progressIndicatorMethod] === 'function') {
       this[progressIndicatorMethod].call(this);
       $(this.element).after(this.progress.element);
     }