android webrtc 创建answer完成，调用setLocalDescription时候报的错

Question

answer的sdp是这样的：

v=0
o=- 4951398896735113745 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE video
a=msid-semantic: WMS
m=video 9 UDP/TLS/RTP/SAVPF 0
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:PMaQ
a=ice-pwd:lEB2R51vV0XiI3srxQvdamhj
a=ice-options:trickle renomination
a=fingerprint:sha-256 EB:F2:51:7D:DA:91:7B:CB:21:A7:6E:E1:23:7C:79:30:56:66:15:F0:22:B6:6D:D1:8E:EA:37:8D:D4:10:17:1F
a=setup:active
a=mid:video
a=recvonly
a=rtcp-mux

setLocalDescription后报的错是这样的：

I/channel.cc: (line 1012): Setting local video description for {mid: video, media_type: video}
I/webrtc_video_engine.cc: (line 1157): SetRecvParameters: {codecs: [], extensions: []}
E/webrtc_video_engine.cc: (line 205): Setting codecs without a video codec is invalid: {}
E/peer_connection.cc: (line 6009): Failed to set local video description recv parameters for m-section with mid='video'. (INVALID_PARAMETER)
(line 2602): Failed to set local answer sdp: Failed to set local video description recv parameters for m-section with mid='video'.

answer的sdp里好像是少了a=rtpmap的配置，但是不知道怎么样才能让创建的answer正常。