七牛视频拼接和转码同时进行时报错 stream mismatched

Question

七牛视频拼接和转码同时进行时报错
Id: z0.57bac87c7823de7b57c8132b
Code: 3
Desc: The fop is failed
InputBucket: mochatest
InputKey: 88_2833545_2016-08-22-11-48-43_2016-08-22-13-18-43.mp4
Pipeline: 1378265990.hls
Reqid: t28AALrfFs_KF20U

    Cmd:    avconcat/2/format/mp4/文件1的base64码/文件2的base64码|avthumb/m3u8/r/13/vb/800k/segtime/10/wmImage/aHR0cDovLzd

qcHR4ZC5jb20yLnowLmdsYi5xaW5pdWNkbi5jb20vd21JbWFnZTIucG5n/wmGravity/NorthWest

    Code:   3
    Desc:   The fop is failed
    Error:  execute fop cmd failed: The 2 audio stream mismatched with basic
    

请问是什么问题?