diff --git a/src/eloquent.php b/src/eloquent.php
index 486a72c..60959ad 100644
--- a/src/eloquent.php
+++ b/src/eloquent.php
@@ -55,6 +55,12 @@ function table_name($model): string
     return $model->getTable();
 }
 
+/**
+ * Get the primary key value from Eloquent model.
+ * 
+ * @param  \Illuminate\Database\Eloquent\Model|null  $model
+ * @return int|string|null
+ */
 function safe_key(?Model $model)
 {
     if (\is_null($model)) {