diff --git a/appveyor.yml b/appveyor.yml
index 655ab56..23dd82e 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -29,7 +29,7 @@ environment:
     GROUP: "EXPERIMENTAL"
     BOOST_ROOT: "C:/Libraries/boost_1_67_0"
     PLATFORM: "win64"
-    REF_BUILD_ID:
+    REF_BUILD_ID: "220dev5"
 
 # called before repo cloning
 init: