What could be the problem with ffmpeg filters?

Question

fmpeg

Good day.
I ran into a problem that when I try to merge a video into a mosaic video, I get the following content error

[abuffer @ 0x158dd80] Unable to parse option value "(null)" as sample format
Last message repeated 1 times
[abuffer @ 0x158dd80] Error setting option sample_fmt to value (null).
[graph 1 input from stream 0:1 @ 0x158dc60] Error applying options to the filter.
Error opening filters!

The command I'm trying to run:

ffmpeg -i 1.webm -i 2.webm -i 3.webm -i 4.webm -filter_complex "nullsrc=size=640x480 [base]; [0:v] setpts=PTS-STARTPTS, scale=320x240 [upperleft]; [1:v] setpts=PTS-STARTPTS, scale=320x240 [upperright]; [2:v] setpts=PTS-STARTPTS, scale=320x240 [lowerleft]; [3:v] setpts=PTS-STARTPTS, scale=320x240 [lowerright]; [base][upperleft] overlay=shortest=1 [tmp1]; [tmp1][upperright] overlay=shortest=1:x=320 [tmp2]; [tmp2][lowerleft] overlay=shortest=1:y=240 [tmp3]; [tmp3][lowerright] overlay=shortest=1:x=320:y=240" -c:v libx264 output.mkv

There is little experience with ffmpeg, and Google has not led to anything, maybe someone will tell you which way to dig?