Why can't I put C25K+ as a tag on my blog or question?

[christian1]

The tag box will not accept the '+' sign.

[JR21]

I don't know the answer to that but I'm just about to meet the chaps from HealthUnlocked so will ask.

[pingle Graduate in reply to JR21]

I tried this too and it wouldn't let me either. + sign probably means something else in the programming world!