diff --git a/build-constraints.yaml b/build-constraints.yaml
index 88b7b0298..960a40052 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -5245,7 +5245,7 @@ packages:
 
     "Sebastian Graf <sgraf1337@gmail.com> @sgraf812":
         - happy-lib
-        - happy # 1.21 # Note, cannot use 1.21.0, and 2.0 blocked by https://github.com/commercialhaskell/stackage/issues/7520
+        - happy < 1.21 # Note, cannot use 1.21.0, and 2.0 blocked by https://github.com/commercialhaskell/stackage/issues/7520
 
     "Grandfathered dependencies":
         - BiobaseNewick