diff --git a/build-constraints.yaml b/build-constraints.yaml
index 0eae9493..f80f81a1 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -749,6 +749,8 @@ package-flags:
     # GHC 7.10 remove
     aeson:
         old-locale: true
+    tttool:
+        old-locale: true
 
     hxt:
         network-uri: true