diff --git a/src/Contracts/ResourceResolver.php b/src/Contracts/ResourceResolver.php
index 4510195..a1ca70e 100644
--- a/src/Contracts/ResourceResolver.php
+++ b/src/Contracts/ResourceResolver.php
@@ -4,5 +4,5 @@
 
 interface ResourceResolver
 {
-    public function resolve(string $model): ?string;
+    public function resolve(?string $model): ?string;
 }