diff --git a/build-constraints.yaml b/build-constraints.yaml
index ea97d449..9f4d4edc 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -6727,6 +6727,9 @@ packages:
       # https://github.com/commercialhaskell/stackage/issues/6237
       - lucid < 2.9.13
 
+      # https://github.com/commercialhaskell/stackage/issues/6242
+      - happy < 1.21
+
 # end of packages
 
 # Package flags are applied to individual packages, and override the values of