diff --git a/src/Relations/BelongsToThrough.php b/src/Relations/BelongsToThrough.php
index 1d1af86..8518504 100644
--- a/src/Relations/BelongsToThrough.php
+++ b/src/Relations/BelongsToThrough.php
@@ -106,7 +106,7 @@ protected function performJoins(Builder $query = null)
     /**
      * Get the foreign key for a model.
      *
-     * @param \Illuminate\Database\Eloquent\Model $model
+     * @param \Illuminate\Database\Eloquent\Model|null $model
      * @return string
      */
     public function getForeignKeyName(Model $model = null)