diff --git a/proxy/http2/Http2ConnectionState.cc b/proxy/http2/Http2ConnectionState.cc
index e06b5fdaefa..a486d4567cf 100644
--- a/proxy/http2/Http2ConnectionState.cc
+++ b/proxy/http2/Http2ConnectionState.cc
@@ -1541,6 +1541,7 @@ Http2ConnectionState::send_a_data_frame(Http2Stream *stream, size_t &payload_len
 
   if (this->ua_session->write_avail() == 0) {
     Http2StreamDebug(this->ua_session, stream->get_id(), "Not write avail");
+    this->ua_session->flush();
     return Http2SendDataFrameResult::NOT_WRITE_AVAIL;
   }