diff --git a/control/Controller.php b/control/Controller.php
index 29abfc30c..025fb00a6 100644
--- a/control/Controller.php
+++ b/control/Controller.php
@@ -363,7 +363,7 @@ class Controller extends RequestHandler implements TemplateGlobalProvider {
 		if(isset($this->requestParams['formController'])) {
 			$formController = Director::getControllerForURL($this->requestParams['formController']);
 
-			while(is_a($formController, 'NestedController')) {
+			while ($formController instanceof NestedController) {
 				$formController = $formController->getNestedController();
 			}
 			return $formController;
diff --git a/control/NestedController.php b/control/NestedController.php
new file mode 100644
index 000000000..1ced32138
--- /dev/null
+++ b/control/NestedController.php
@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * Interface that is implemented by controllers that are designed to hand control over to another controller.
+ * ModelAsController, which selects up a SiteTree object and passes control over to a suitable subclass of ContentController, is a good
+ * example of this.
+ *
+ * Controllers that implement this interface must always return a nested controller.
+ */
+interface NestedController {
+
+	/**
+	 * Get overriding controller
+	 *
+	 * @return Controller
+	 */
+	public function getNestedController();
+
+}