diff --git a/src/libsysprof/sysprof-control-source.c b/src/libsysprof/sysprof-control-source.c
index 04d8ec38..4ccdc482 100644
--- a/src/libsysprof/sysprof-control-source.c
+++ b/src/libsysprof/sysprof-control-source.c
@@ -142,6 +142,8 @@ event_frame_cb (gconstpointer  data,
 
   _sysprof_capture_writer_add_raw (rd->self->writer, fr);
 
+  *length = fr->len;
+
   return G_SOURCE_CONTINUE;
 
 remove_source: