diff --git a/lib_eio/unix/eio_unix.mli b/lib_eio/unix/eio_unix.mli
index 774ba5f6..1b94254c 100644
--- a/lib_eio/unix/eio_unix.mli
+++ b/lib_eio/unix/eio_unix.mli
@@ -58,7 +58,7 @@ val run_in_systhread : ?label:string -> (unit -> 'a) -> 'a
 
     @param label The operation name to use in trace output. *)
 
-val pipe : Switch.t -> source_ty r * sink_ty r
+val pipe : Switch.t -> [< source_ty] r * [< sink_ty] r
 (** [pipe sw] returns a connected pair of flows [src] and [sink]. Data written to [sink]
     can be read from [src].
     Note that, like all FDs created by Eio, they are both marked as close-on-exec by default. *)