diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 09621b2..e329ea7 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -490,8 +490,8 @@ else if (this.progress.type === 'throbber') {
     // Reattach behaviors, if they were detached in beforeSerialize(). The
     // attachBehaviors() called on the new content from processing the response
     // commands is not sufficient, because behaviors from the entire form need
-    // to be reattached.
-    if (this.$form) {
+    // to be reattached, if it's still part of the document.
+    if (this.form && $.contains(document, this.form)) {
       var settings = this.settings || drupalSettings;
       Drupal.attachBehaviors(this.$form.get(0), settings);
     }
@@ -543,8 +543,9 @@ else if (type === 'fade') {
     $(this.wrapper).show();
     // Re-enable the element.
     $(this.element).removeClass('progress-disabled').prop('disabled', false);
-    // Reattach behaviors, if they were detached in beforeSerialize().
-    if (this.$form) {
+    // Reattach behaviors, if they were detached in beforeSerialize(), and the
+    // form is still part of the document.
+    if (this.form && $.contains(document, this.form)) {
       var settings = response.settings || this.settings || drupalSettings;
       Drupal.attachBehaviors(this.$form.get(0), settings);
     }