'$1: unbound variable' in the line 'case "$1" in'

[ehula]

I can run the script from the command line just fine. However, I am running it on NixOS, and am trying to run it using an overlay. When I do so, I get the following error:
$1: unbound variable

refering to this line
case "$1" in

I think it is a bug with the script, and not the setup in NixOS.

[plushrib]

this is extremly late but put the script in your $PATH and install bc