diff --git a/capnp-rpc-unix.opam b/capnp-rpc-unix.opam
index b23ed902..5cab7c43 100644
--- a/capnp-rpc-unix.opam
+++ b/capnp-rpc-unix.opam
@@ -22,7 +22,7 @@ depends: [
   "alcotest" {>= "1.0.1" & with-test}
   "mirage-crypto-rng" {>= "0.11.0"}
   "mirage-crypto-rng-lwt" {>= "0.11.0"}
-  "mdx" {with-test}
+  "mdx" {>= "2.2.0" & < "2.4.0" & with-test}
   "asetmap" {with-test}
   "eio_main"
 ]