diff --git a/src/MapsFactory.php b/src/MapsFactory.php
index 68c98c5d..498f7d49 100644
--- a/src/MapsFactory.php
+++ b/src/MapsFactory.php
@@ -219,7 +219,7 @@ public function getParamDefinitionFactory(): ParamDefinitionFactory {
 		return $factory;
 	}
 
-	public function newGeoJsonFetcher( FileFetcher $fileFetcher = null ): GeoJsonFetcher {
+	public function newGeoJsonFetcher( ?FileFetcher $fileFetcher = null ): GeoJsonFetcher {
 		return new GeoJsonFetcher(
 			$fileFetcher ?? $this->getGeoJsonFileFetcher(),
 			$this->mediaWikiServices->getTitleParser(),