diff --git a/specification/schemas/hooks/HookActionRejectReturn.json b/specification/schemas/hooks/HookActionRejectReturn.json
index aeb896f4..e425010c 100644
--- a/specification/schemas/hooks/HookActionRejectReturn.json
+++ b/specification/schemas/hooks/HookActionRejectReturn.json
@@ -14,7 +14,10 @@
         "additionalProperties": false,
         "properties": {
           "message": {
-            "type": "string",
+            "type": [
+              "string",
+              "null"
+            ],
             "maxLength": 255,
             "description": "Message explaining the reason for rejection.",
             "example": "The items you've selected are not eligible to be returned because..."