[FFmpeg-user] Stream specifier ':a' in filtergraph Error

[Randy Johnson]

Hello,

When running the following command:

```
ffmpeg \
-loop 1 -framerate 30 -t 1.4 -i /assets/img/filler640480.jpg -i 0d.mp4 -
y \
-loop 1 -framerate 30 -t 891.113 -i /assets/img/filler640480.jpg -i
9f.mp4 -y \
-f lavfi -t 0.1 -i anullsrc=channel_layout=mono:sample_rate=48000 \
-filter_complex "
[0:v]setsar=1[v0];[2:v]setsar=1[v1];[v0][4:a][1:v][1:a][v1][4:a][3:v][3:a]concat=n=4:v=1:a=1"
-vsync 2 -vcodec libx264 -pix_fmt yuv420p 0245-grid.mp4

```

I am getting the following error:

```
Stream specifier ':a' in filtergraph description 
[0:v]setsar=1[v0];[2:v]setsar=1[v1];[v0][4:a][1:v][1:a][v1][4:a][3:v][3:a]concat=n=4:v=1:a=1
matches no streams.
```

This only happens on some video processing not all, cannot seem to
figure out why.

Thank You,

Randy