ffmpeg 处理只有声音没有图像的mp4文件，添加水印会报错

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命令：

/usr/bin/ffmpeg -i /data/file/G1iWlrvS/G1iWlrvS.mp4 -i /data/watermark/SipEdu.png -threads 4 -filter_complex overlay=main_w-overlay_w-16:16 -vcodec libx264 -acodec copy -max_muxing_queue_size 1024 /data/file/G1iWlrvS/wm-G1iWlrvS.mp4

ffmpeg version 3.4.8 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 4.8.5 (GCC) 20150623 (Red Hat 4.8.5-44)
  configuration: --prefix=/data/ffmpeg --enable-libmp3lame --enable-gpl --enable-version3 --enable-nonfree --enable-pthreads --enable-libfdk-aac --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libx264 --enable-libxvid --enable-postproc --enable-ffserver --enable-ffplay --enable-zlib
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/data/file/G1iWlrvS/G1iWlrvS.mp4':
  Metadata:
    major_brand     : M4A 
    minor_version   : 0
    compatible_brands: M4A mp42isom
    creation_time   : 2020-12-30T14:47:20.000000Z
    iTunSMPB        :  00000000 00000840 00000000 000000000062B7C0 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  Duration: 00:02:26.70, start: 0.047891, bitrate: 309 kb/s
    Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 308 kb/s (default)
    Metadata:
      creation_time   : 2020-12-30T14:47:20.000000Z
      handler_name    : Core Media Audio
Input #1, png_pipe, from '/data/watermark/SipEdu.png':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Video: png, rgba(pc), 125x47 [SAR 2835:2835 DAR 125:47], 25 tbr, 25 tbn, 25 tbc
Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_overlay_0

会出现如下错误

Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_overlay_0

那么问题来了 对于只有声音没有画面的mp4视频文件该如何添加水印呢？