diff --git a/src/h2load_quic.cc b/src/h2load_quic.cc
index daae278d..86c85a4f 100644
--- a/src/h2load_quic.cc
+++ b/src/h2load_quic.cc
@@ -593,10 +593,10 @@ int Client::write_quic() {
 
       quic.last_error = quic::err_transport(nwrite);
       return -1;
+    } else if (ndatalen > 0 && s->add_write_offset(stream_id, ndatalen) != 0) {
+      return -1;
     }
 
-    assert(ndatalen == -1);
-
     quic_restart_pkt_timer();
 
     if (nwrite == 0) {