diff --git a/riot/lib/gen_server.ml b/riot/lib/gen_server.ml
index f79b5ead..685305de 100644
--- a/riot/lib/gen_server.ml
+++ b/riot/lib/gen_server.ml
@@ -16,7 +16,7 @@ module type Impl = sig
   val init : args -> state init_result
   val handle_call : 'res. 'res req -> Pid.t -> state -> 'res
   val handle_info : Message.t -> state -> unit
-  val handle_cast : 'res req -> state -> unit
+  val handle_cast : 'res. 'res req -> state -> unit
 end
 
 type ('args, 'state) impl =