diff --git a/lib/includes/Formatters/MwTimeIsoFormatter.php b/lib/includes/Formatters/MwTimeIsoFormatter.php
index 2161ec336d..3d5b95cbb9 100644
--- a/lib/includes/Formatters/MwTimeIsoFormatter.php
+++ b/lib/includes/Formatters/MwTimeIsoFormatter.php
@@ -150,7 +150,7 @@ private function getDayFormat( $dateFormat ) {
 	 *
 	 * @return string A date format for the month that roundtrips the Wikibase TimeParsers.
 	 */
-	private function getMonthFormat( $dateFormat ) {
+	private function getMonthFormat( ?string $dateFormat ): string {
 		if ( $dateFormat && preg_match( '/(?:[FMn]|(?<!x)m|xg)[.,]?/', $dateFormat, $matches ) ) {
 			return $matches[0];
 		}