diff --git a/wp-includes/meta.php b/wp-includes/meta.php
index cd684b6..f9b0b3d 100644
--- a/wp-includes/meta.php
+++ b/wp-includes/meta.php
@@ -1139,6 +1139,10 @@ function unregister_meta_key( $object_type, $meta_key ) {
 function get_registered_meta_keys( $object_type ) {
 	global $wp_meta_keys;
 
+	if ( ! is_array( $wp_meta_keys ) ) {
+		return array();
+	}
+
 	if ( ! isset( $wp_meta_keys[ $object_type ] ) ) {
 		return array();
 	}