[FFmpeg-user] AAC track copy problem in mp4

[Petr Tresnak]

Hi all,
I downscale the following 4k to 1080 using ffmpeg -i 4k.mp4 -vf scale=1920:1080 -c:v libx264 -preset fast -crf 18 -c:a copy output.mp4"

Duration: 00:00:10.56, start: 0.080000, bitrate: 95904 kb/
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709), 3840x2160 [SAR 1:1 DAR 16:9], 95655 kb/s, 25 fps
Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 124 kb/s (default)

But there is a problem with proper copy of audio track:

[mp4 @ 05e30860] track 1: codec frame size is not set
[mp4 @ 05e30860] Packet with invalid duration -1024 in stream 1

In trans-coded video file there is a small difference in audio afterwards (AAC vs AAC LC, bitrate 124 vs 125):
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 125 kb/s (default)

Can you help how to force ffmpeg to copy audio track?
Thanks
Peter