diff --git a/build-constraints.yaml b/build-constraints.yaml
index d2831c33..84496855 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -6703,6 +6703,9 @@ packages:
       - megaparsec < 9.1
       - megaparsec-tests < 9.1
 
+      # https://github.com/commercialhaskell/stackage/issues/6122
+      - mmorph < 1.2
+
 # end of packages
 
 # Package flags are applied to individual packages, and override the values of