I happened to write by mistake:

-dGraphicsAlphaBits=4 -dTextAlphaBit=n

with a literal 'n' instead of replacing it with a number.

The error message that I got was:

-dvar=name requires name=null, true, or false

This error message doesn't make any sense to the user.

A correct error message would be something like 
"invalid value 'n' for option -dTextAlphaBit"

Note there are two issues:
1. the error message doesn't make sense to the user (why null, true or false?) (I do understand, I guess, where this comes from, but this is too low-level)
2. it doesn't tell you WHICH option it is complaining about.

Issue 2 is very important too. I spent several minutes staring at the -dGraphicsAlphaBit=4 and wondering what was wrong with the 4.