stream with mprgtsmux

[James]

As usual I'm way over my head. If anyone can help then please.

My long term goal is to send video and audio as a muxed ts stream.
From rstp-server examples test-video.c

This pipeline works without mpegtsmux

 gst_rtsp_media_factory_set_launch (factory, "( "
      "v4l2src device=/dev/video2 ! video/x-h264,width=1920,height=1080,framerate=30/1 ! "
      "mpegtsmux name=mux ! rtph264pay name=pay0 pt=96 "
      ")");

but with mpegtsmux

 0:00:11.276990231 22905 0x7f33f8003b60 ERROR           GST_PIPELINE gst/parse/grammar.y:779:gst_parse_perform_link: could not link mux to pay0

mux can source 
video/x-h264
          stream-format: byte-stream
while rtph264pay can sink
      video/x-h264
          stream-format: byte-stream

(my camera gives h264) and I don't understand
James