diff --git a/tox.ini b/tox.ini
index b44ef0b08a7..65ed82b385b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -159,6 +159,7 @@ commands = {toxinidir}/bin/admin/build-apidocs {toxinidir}/src/ apidocs
 description = run Mypy (static type checker)
 
 extras = mypy
+# https://github.com/twisted/incremental/pull/69
 deps = incremental @ https://github.com/twisted/incremental/archive/refs/heads/trunk.tar.gz
 commands =
     mypy                                       \