Encoding with non-8/16/24 bits per sample with flac

Hi everyone,
I'm trying to generate flac files with various bits-per-samples, using 
the 'flac' reference encoder and a raw audio file (I'm writing a decoder 
in Java and I need to create various test files).
Creating 8, 16 or 24 bps works fine, but I didn't find how to generate 
other bps; the "--bps" option seems to accept only 8,16 or24.
Any idea ?

Thanks in advance.