diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index c524b06928..576b3c657a 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -2137,8 +2137,15 @@ function sanitize_user( $username, $strict = false ) {
  */
 function sanitize_key( $key ) {
 	$raw_key = $key;
-	$key     = strtolower( $key );
-	$key     = preg_replace( '/[^a-z0-9_\-]/', '', $key );
+
+	if ( ! is_string( $key ) ) {
+		$key = '';
+	}
+
+	if ( '' !== $key ) {
+		$key = strtolower( $key );
+		$key = preg_replace( '/[^a-z0-9_\-]/', '', $key );
+	}
 
 	/**
 	 * Filters a sanitized key string.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index a720a13e36..d890e34c51 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52291';
+$wp_version = '5.9-alpha-52292';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.