ezmlm-issubn: non-existent directories

Hi,

I am just about to move my old v0.435 setup of ezmlm-idx to v6.0.1.

Meanwhile I stumbled over the following behaviour of ezmlm-issubn in v6:

the command 
 ezmlm-issubn '/data/lists/foo'; echo $?
returns:
 0

the command
 ezmlm-issubn '/data/lists/foo' '/data/lists/foo/digest'; echo $?
returns:
 99

(note: the 'digest' directory does not exist)

I compared the behaviour of v0.435: it returns "0" for both combinations of
arguments.

The behaviour of v0.435 is what I would expect, since it should be sufficient,
that the SENDER address is in any of the given (list-)directories.

how can I fix this difference here?

thanks for your time,
Lars