diff --git a/src/SortableTrait.php b/src/SortableTrait.php
index 55c0d16..d8cc172 100644
--- a/src/SortableTrait.php
+++ b/src/SortableTrait.php
@@ -44,8 +44,8 @@ public function scopeOrdered(Builder $query, string $direction = 'asc')
     public static function setNewOrder(
         $ids,
         int $startOrder = 1,
-        string $primaryKeyColumn = null,
-        callable $modifyQuery = null
+        ?string $primaryKeyColumn = null,
+        ?callable $modifyQuery = null
     ): void {
         if (! is_array($ids) && ! $ids instanceof ArrayAccess) {
             throw new InvalidArgumentException('You must pass an array or ArrayAccess object to setNewOrder');