diff --git a/lib/ethif.ml b/lib/ethif.ml
index a3ebcd963..59c822356 100644
--- a/lib/ethif.ml
+++ b/lib/ethif.ml
@@ -49,9 +49,9 @@ module Make(Netif : V1_LWT.NETWORK) = struct
         | Some Wire_structs.ARP -> arpv4 frame
         | Some Wire_structs.IPv4 -> ipv4 payload
         | Some Wire_structs.IPv6 -> ipv6 payload
-        | None -> return_unit (* TODO: default ethertype payload handler *)
+        | None -> Lwt.return_unit (* TODO: default ethertype payload handler *)
       end
-    | _ -> return_unit
+    | _ -> Lwt.return_unit
 
   let write t frame =
     MProf.Trace.label "ethif.write";