diff --git a/src/wp-includes/capabilities-functions.php b/src/wp-includes/capabilities-functions.php
index 3742c18c40..0c1be98c1d 100644
--- a/src/wp-includes/capabilities-functions.php
+++ b/src/wp-includes/capabilities-functions.php
@@ -53,6 +53,12 @@ function map_meta_cap( $cap, $user_id ) {
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		if ( ! $post_type ) {
+			/* translators: 1: post type, 2: capability name */
+			_doing_it_wrong( __FUNCTION__, sprintf( __( 'The post type %1$s is not registered, so it may not be reliable to check the capability "%2$s" against a post of that type.' ), $post->post_type, $cap ), '4.4.0' );
+			$caps[] = 'edit_others_posts';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -101,6 +107,12 @@ function map_meta_cap( $cap, $user_id ) {
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		if ( ! $post_type ) {
+			/* translators: 1: post type, 2: capability name */
+			_doing_it_wrong( __FUNCTION__, sprintf( __( 'The post type %1$s is not registered, so it may not be reliable to check the capability "%2$s" against a post of that type.' ), $post->post_type, $cap ), '4.4.0' );
+			$caps[] = 'edit_others_posts';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -143,6 +155,12 @@ function map_meta_cap( $cap, $user_id ) {
 		}
 
 		$post_type = get_post_type_object( $post->post_type );
+		if ( ! $post_type ) {
+			/* translators: 1: post type, 2: capability name */
+			_doing_it_wrong( __FUNCTION__, sprintf( __( 'The post type %1$s is not registered, so it may not be reliable to check the capability "%2$s" against a post of that type.' ), $post->post_type, $cap ), '4.4.0' );
+			$caps[] = 'edit_others_posts';
+			break;
+		}
 
 		if ( ! $post_type->map_meta_cap ) {
 			$caps[] = $post_type->cap->$cap;
@@ -169,6 +187,12 @@ function map_meta_cap( $cap, $user_id ) {
 	case 'publish_post':
 		$post = get_post( $args[0] );
 		$post_type = get_post_type_object( $post->post_type );
+		if ( ! $post_type ) {
+			/* translators: 1: post type, 2: capability name */
+			_doing_it_wrong( __FUNCTION__, sprintf( __( 'The post type %1$s is not registered, so it may not be reliable to check the capability "%2$s" against a post of that type.' ), $post->post_type, $cap ), '4.4.0' );
+			$caps[] = 'edit_others_posts';
+			break;
+		}
 
 		$caps[] = $post_type->cap->publish_posts;
 		break;
diff --git a/tests/phpunit/tests/user/capabilities.php b/tests/phpunit/tests/user/capabilities.php
index 78820d2f9d..aa0c16af47 100644
--- a/tests/phpunit/tests/user/capabilities.php
+++ b/tests/phpunit/tests/user/capabilities.php
@@ -994,4 +994,27 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 
 		$this->assertFalse( current_user_can( 'edit_user', $super_admin->ID ) );
 	}
+
+	/**
+	 * @ticket 16956
+	 */
+	function test_require_edit_others_posts_if_post_type_doesnt_exist() {
+		register_post_type( 'existed' );
+		$post_id = $this->factory->post->create( array( 'post_type' => 'existed' ) );
+		_unregister_post_type( 'existed' );
+
+		$subscriber_id = $this->factory->user->create( array( 'role' => 'subscriber' ) );
+		$editor_id = $this->factory->user->create( array( 'role' => 'editor' ) );
+
+		$this->setExpectedIncorrectUsage( 'map_meta_cap' );
+		foreach ( array( 'delete_post', 'edit_post', 'read_post', 'publish_post' ) as $cap ) {
+			wp_set_current_user( $subscriber_id );
+			$this->assertSame( array( 'edit_others_posts' ), map_meta_cap( $cap, $subscriber_id, $post_id ) );
+			$this->assertFalse( current_user_can( $cap, $post_id ) );
+
+			wp_set_current_user( $editor_id );
+			$this->assertSame( array( 'edit_others_posts' ), map_meta_cap( $cap, $editor_id, $post_id ) );
+			$this->assertTrue( current_user_can( $cap, $post_id ) );
+		}
+	}
 }