Can't only use optional arguments?

[ergpopler]

Example of what should be done:

./prog.lua build

and

./prog.lua install

but if i make install and build both options i get the following error:
mpm: bad number of arguments: 0 argument(s) must be specified, not 1; re-run with help for usage
I can set build or install as arguments, but that makes both options REQUIRED, which is not what is wanted.