allm on "pre_comment_approved, return values"

I am using the pre_comment_approved filter and it seems the return value of 0 does not function. A return value of 'spam' does what can be expected.

Any ideas?

This is what the codex says:
http://codex.wordpress.org/Plugin_API/Filter_Reference/pre_comment_approved

I did some checking:

A return value of '0' (not a defined return value!) does not work, which is fine.

A return value of false (not a defined return value!) has the result that the comment is not shown in the admin, while it is still in the database. There are 2 e-mails to moderate.