diff --git a/wp-includes/compat.php b/wp-includes/compat.php
index 6fe5ab37e1..b95275455c 100644
--- a/wp-includes/compat.php
+++ b/wp-includes/compat.php
@@ -79,6 +79,10 @@ endif;
  * @return string Extracted substring.
  */
 function _mb_substr( $str, $start, $length = null, $encoding = null ) {
+	if ( null === $str ) {
+		return '';
+	}
+
 	if ( null === $encoding ) {
 		$encoding = get_option( 'blog_charset' );
 	}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 437a74f915..5ad69fbe79 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51852';
+$wp_version = '5.9-alpha-51853';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.