diff --git a/build-constraints.yaml b/build-constraints.yaml
index 52a5b002..d91bdc29 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -3331,6 +3331,9 @@ packages:
 
         # https://github.com/fpco/stackage/issues/2661
         - servant-docs < 0.11
+
+        # https://github.com/fpco/stackage/issues/2666
+        - megaparsec < 6.0
 # end of packages
 
 # Package flags are applied to individual packages, and override the values of