diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
index 4fce45878c..3dc8d62d1b 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
@@ -1592,7 +1592,13 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 	 * @return bool Whether post can be read.
 	 */
 	protected function check_read_post_permission( $post, $request ) {
-		$post_type        = get_post_type_object( $post->post_type );
+		$post_type = get_post_type_object( $post->post_type );
+
+		// Return false if custom post type doesn't exist
+		if ( ! $post_type ) {
+			return false;
+		}
+
 		$posts_controller = $post_type->get_rest_controller();
 
 		// Ensure the posts controller is specifically a WP_REST_Posts_Controller instance
diff --git a/tests/phpunit/tests/rest-api/rest-comments-controller.php b/tests/phpunit/tests/rest-api/rest-comments-controller.php
index da2d372a54..46738edc72 100644
--- a/tests/phpunit/tests/rest-api/rest-comments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-comments-controller.php
@@ -3240,4 +3240,31 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 			$this->assertArrayNotHasKey( 'raw', $data['content'] );
 		}
 	}
+
+	/**
+	 * @ticket 42238
+	 */
+	public function test_check_read_post_permission_with_invalid_post_type() {
+		register_post_type(
+			'bug-post',
+			array(
+				'label'        => 'Bug Posts',
+				'supports'     => array( 'title', 'editor', 'author', 'comments' ),
+				'show_in_rest' => true,
+				'public'       => true,
+			)
+		);
+		create_initial_rest_routes();
+
+		$post_id    = self::factory()->post->create( array( 'post_type' => 'bug-post' ) );
+		$comment_id = self::factory()->comment->create( array( 'comment_post_ID' => $post_id ) );
+		_unregister_post_type( 'bug-post' );
+
+		$this->setExpectedIncorrectUsage( 'map_meta_cap' );
+
+		wp_set_current_user( self::$admin_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/comments/' . $comment_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertEquals( 403, $response->get_status() );
+	}
 }