diff --git a/src/wp-includes/class-wp-post-type.php b/src/wp-includes/class-wp-post-type.php
index 0099515050..bb148067c0 100644
--- a/src/wp-includes/class-wp-post-type.php
+++ b/src/wp-includes/class-wp-post-type.php
@@ -343,7 +343,7 @@ final class WP_Post_Type {
 	 * @since 5.3.0
 	 * @var WP_REST_Controller $rest_controller
 	 */
-	private $rest_controller;
+	public $rest_controller;
 
 	/**
 	 * Constructor.
@@ -722,6 +722,10 @@ final class WP_Post_Type {
 			$this->rest_controller = new $class( $this->name );
 		}
 
+		if ( ! ( $this->rest_controller instanceof $class ) ) {
+			return null;
+		}
+
 		return $this->rest_controller;
 	}
 }
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 4a00336e34..2b71a76cb0 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -15,15 +15,6 @@
  * @see WP_REST_Controller
  */
 class WP_REST_Posts_Controller extends WP_REST_Controller {
-
-	/**
-	 * Instances of post type controllers keyed by post type.
-	 *
-	 * @since 5.3.0
-	 * @var WP_REST_Controller[]
-	 */
-	private static $post_type_controllers = array();
-
 	/**
 	 * Post type.
 	 *
diff --git a/tests/phpunit/tests/rest-api/rest-posts-controller.php b/tests/phpunit/tests/rest-api/rest-posts-controller.php
index d1a30c69a4..37ee1ac02c 100644
--- a/tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -4617,6 +4617,37 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		);
 	}
 
+	/**
+	 * @ticket 45677
+	 */
+	public function test_get_for_post_type_returns_null_for_invalid_provided_controller() {
+		register_post_type(
+			'test',
+			array(
+				'show_in_rest'    => true,
+				'rest_controller' => new \stdClass(),
+			)
+		);
+
+		$this->assertNull( get_post_type_object( 'test' )->get_rest_controller() );
+	}
+
+	/**
+	 * @ticket 45677
+	 */
+	public function test_get_for_post_type_returns_null_for_controller_class_mismatch() {
+		register_post_type(
+			'test',
+			array(
+				'show_in_rest'          => true,
+				'rest_controller_class' => WP_REST_Posts_Controller::class,
+				'rest_controller'       => new WP_REST_Terms_Controller( 'category' ),
+			)
+		);
+
+		$this->assertNull( get_post_type_object( 'test' )->get_rest_controller() );
+	}
+
 	public function tearDown() {
 		_unregister_post_type( 'private-post' );
 		_unregister_post_type( 'youseeme' );