diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 3d1227ff9a..1e5d3a27f0 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -1342,9 +1342,9 @@ function register_post_type( $post_type, $args = array() ) {
 	$post_type = sanitize_key( $post_type );
 	$args->name = $post_type;
 
-	if ( strlen( $post_type ) > 20 ) {
-		_doing_it_wrong( __FUNCTION__, __( 'Post types cannot exceed 20 characters in length' ), '4.0' );
-		return new WP_Error( 'post_type_too_long', __( 'Post types cannot exceed 20 characters in length' ) );
+	if ( empty( $post_type ) || strlen( $post_type ) > 20 ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Post type names must be between 1 and 20 characters in length.' ), '4.2' );
+		return new WP_Error( 'post_type_length_invalid', __( 'Post type names must be between 1 and 20 characters in length.' ) );
 	}
 
 	// If not set, default to the setting for public.
diff --git a/tests/phpunit/tests/post/types.php b/tests/phpunit/tests/post/types.php
index 0c8d95d298..e1582d5d8d 100644
--- a/tests/phpunit/tests/post/types.php
+++ b/tests/phpunit/tests/post/types.php
@@ -19,6 +19,26 @@ class Tests_Post_Types extends WP_UnitTestCase {
 		_unregister_post_type( 'foo' );
 	}
 
+	/**
+	 * @ticket 31134
+	 *
+	 * @expectedIncorrectUsage register_post_type
+	 */
+	function test_register_post_type_length_long() {
+		// post type too long
+		$this->assertInstanceOf( 'WP_Error', register_post_type( 'abcdefghijklmnopqrstuvwxyz0123456789' ) );
+	}
+
+	/**
+	 * @ticket 31134
+	 *
+	 * @expectedIncorrectUsage register_post_type
+	 */
+	function test_register_post_type_length_short() {
+		// post type too short
+		$this->assertInstanceOf( 'WP_Error', register_post_type( '' ) );
+	}
+
 	function test_register_taxonomy_for_object_type() {
 		global $wp_taxonomies;