diff --git a/packages/custom-client/src/core/params.ts b/packages/custom-client/src/core/params.ts
index 7559bbb8c..07bd5dde3 100644
--- a/packages/custom-client/src/core/params.ts
+++ b/packages/custom-client/src/core/params.ts
@@ -31,7 +31,7 @@ type KeyMap = Map<
   string,
   {
     in: Slot;
-    map?: string;
+    map?: string | undefined;
   }
 >;