Module used by -1

[Daniel Nichter]

What does it mean when a module is used by -1?

Example output from lsmod:
Module                  Size  Used by
zftape                 56864  -1

I'm using Slackware Linux 2.2.3, modutils 2.1.121 and
that zftape module is the one that came w/ the kernel.

Thanks,
Daniel