diff --git a/src/plugins/wp/prover.ml b/src/plugins/wp/prover.ml
index ab3da9f90b14a6958b80748b73d6a195eb99278a..b92b9cdec54c9c9a0dfb654096a5d0a27fad92bb 100644
--- a/src/plugins/wp/prover.ml
+++ b/src/plugins/wp/prover.ml
@@ -76,7 +76,7 @@ let run_prover wpo ?config ?(mode=Batch) ?progress ?result prover =
   Task.return (VCS.is_valid res)
 
 let simplify ?start ?result wpo =
-  Task.call
+  Server.Main.async
     (fun wpo ->
        let r = Wpo.get_result wpo VCS.Qed in
        VCS.( r.verdict == Valid ) ||