diff --git a/src/subscriber/callbacks.ml b/src/subscriber/callbacks.ml
index 70bde0f..10ec72a 100644
--- a/src/subscriber/callbacks.ml
+++ b/src/subscriber/callbacks.ml
@@ -88,7 +88,7 @@ end
 
 (** Build a set of callbacks.
     @since NEXT_RELEASE *)
-let make ?(on_init = Dummy.on_init) ?(new_span_id = Dummy.new_span_id)
+let make ~new_span_id ?(on_init = Dummy.on_init)
     ?(on_enter_span = Dummy.on_enter_span) ?(on_exit_span = Dummy.on_exit_span)
     ?(on_message = Dummy.on_message) ?(on_counter = Dummy.on_counter)
     ?(on_extension_event = Dummy.on_extension_event)
@@ -105,4 +105,4 @@ let make ?(on_init = Dummy.on_init) ?(new_span_id = Dummy.new_span_id)
   }
 
 (** Dummy callbacks, ignores all events. *)
-let dummy () : _ t = make ()
+let dummy () : _ t = make ~new_span_id:(fun () -> Span_sub.dummy_span_id) ()
diff --git a/src/subscriber/span_sub.ml b/src/subscriber/span_sub.ml
index 177e225..04dedae 100644
--- a/src/subscriber/span_sub.ml
+++ b/src/subscriber/span_sub.ml
@@ -9,6 +9,8 @@ open Trace_core
 type span_id = int64
 (** Unique ID *)
 
+let dummy_span_id = Int64.min_int
+
 type span_flavor =
   [ `Sync
   | `Async