diff --git a/unity-project-cached/action.yaml b/unity-project-cached/action.yaml
index a1ecc29..10ff944 100644
--- a/unity-project-cached/action.yaml
+++ b/unity-project-cached/action.yaml
@@ -9,8 +9,9 @@ inputs:
     description: "Unity Platform. Options: windows, windows32bit, mac, linux, android"
     required: true
   serial:
-    description: "Unity license serial number."
-    required: false
+    description: "Unity license serial number. Or 'personal' for a personal license."
+    required: true
+    default: personal
   email:
     description: "Unity email."
     required: true