diff --git a/mock-api/rest_server.ts b/mock-api/rest_server.ts
index aa6a0d0b2..3043baf95 100644
--- a/mock-api/rest_server.ts
+++ b/mock-api/rest_server.ts
@@ -14,7 +14,7 @@ const headers = {
   'Content-type': 'application/msgpack'
 };
 
-let VERSION_IS_UPGRADEABLE;
+let VERSION_IS_UPGRADEABLE: boolean;
 
 // Versions
 // default - on latest stable, no upgrades