diff --git a/src/Controller/Component/AjaxComponent.php b/src/Controller/Component/AjaxComponent.php
index e3f994a6..86b932e6 100644
--- a/src/Controller/Component/AjaxComponent.php
+++ b/src/Controller/Component/AjaxComponent.php
@@ -75,7 +75,7 @@ public function beforeRender(Event $event) {
 	}
 
 	/**
-	 * AjaxComponent::respondAsAjax()
+	 * AjaxComponent::_respondAsAjax()
 	 *
 	 * @return void
 	 */
@@ -88,6 +88,11 @@ protected function _respondAsAjax() {
 			$this->Controller->set('_message', $message);
 		}
 
+		// If _serialize is true, *all* viewVars will be serialized; no need to add _message.
+		if (!empty($this->Controller->viewVars['_serialize']) && $this->Controller->viewVars['_serialize'] === true) {
+			return;
+		}
+
 		$serializeKeys = ['_message'];
 		if (!empty($this->Controller->viewVars['_serialize'])) {
 			$serializeKeys = array_merge($serializeKeys, $this->Controller->viewVars['_serialize']);