diff --git a/Processor.php b/Processor.php
index 1c1ff3e..e2564aa 100644
--- a/Processor.php
+++ b/Processor.php
@@ -2232,7 +2232,7 @@ private function elementToRdf(JsonObject $element)
      *
      * @return IRI Returns the IRI of the head of the list
      */
-    private function listToRdf(array $entries, array &$quads, IRI $graph = null)
+    private function listToRdf(array $entries, array &$quads, ?IRI $graph = null)
     {
         if (0 === count($entries)) {
             return new IRI(RdfConstants::RDF_NIL);