You may need a different block size. I'm not really up on my filesystem theory so I don't really remember how it all works. Looking into the code though I found the error:
genext2fs.c#L906.
From what I can read in the code, the block size must be greater than 0, greater than 8 and greater than EXT2_FIRST_INO - 1 + (nbresrvd ? 1 : 0). Where EXT2_FIRST_INO is 11.
nbresrvd is set here:
genext2fs.c#L3346-L3349. We aren't hitting any of those errors though. but I'm not entirely sure what we need to specify.
What you should probably do is figure out the sum of the size of the files you are generating a filesystem out of and set -b and --block-size (-B) appropriately (This can be 1024, 2048 or 4096 bytes).