diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index fefe9f3..7069e72 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -708,7 +708,7 @@
     // when there is a form such that this.$form.ajaxSubmit() is used instead of
     // $.ajax(). When there is no form and $.ajax() is used, beforeSerialize()
     // isn't called, but don't rely on that: explicitly check this.$form.
-    if (this.$form) {
+    if (this.$form && $.contains(document, this.$form.get(0))) {
       var settings = this.settings || drupalSettings;
       Drupal.detachBehaviors(this.$form.get(0), settings, 'serialize');
     }
@@ -957,8 +957,9 @@
     $(this.wrapper).show();
     // Re-enable the element.
     $(this.element).prop('disabled', false);
-    // Reattach behaviors, if they were detached in beforeSerialize().
-    if (this.$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.get(0))) {
       var settings = this.settings || drupalSettings;
       Drupal.attachBehaviors(this.$form.get(0), settings);
     }