what does this warning means?

[Gontzal <gontzalp@xxxxxxxxx>]

Hi list,

I always have this messages on my cache.log, but i've never been
worried about them, it is just curiosity to know what this means and
if I can solve it:

The message is:

2011/02/25 09:53:53| WARNING: HTTP header contains NULL characters
{Accept: */*^M
Content-Type: application/x-www-form-urlencoded}


Thanks a lot