diff --git a/src/signkey.ml b/src/signkey.ml
index 0bdd3b37..67a14a6d 100644
--- a/src/signkey.ml
+++ b/src/signkey.ml
@@ -19,10 +19,7 @@ let generate () =
      correctly set those: *)
   let stamp_start = Ptime_clock.now () in
   let stamp_expire =
-    match Ptime.add_span stamp_start Config.Exchange.signkey_legal_duration with
-    | None ->
-        Fmt.failwith "Ptime.add_span: result is not in the range [min;max]"
-    | Some v -> v
+    Util.ptime_add_span_exn stamp_start Config.Exchange.signkey_legal_duration
   in
   let stamp_end = stamp_expire in
   let priv, pub = Mirage_crypto_ec.Ed25519.generate () in