[Groff] how to break a footnote

[James K. Lowden]

What do you do about long URLs in footnotes?  

.FS
http://db.lcs.mit.edu/projects/cstore/vldb.pdf
.FE

engenders a complaint in a 2C document:

dbformat.ms:983: warning [p 11, 3.6i, div address@hidden', 0.0i]: can't break
line

(I suppose I should be able to read that, but I haven't taken the time
to discover what address@hidden might mean!)  

man 7 groff tells me 

        \:    Inserts  a  zero-width  break point (similar to \% 
            but without a soft hyphen character).

but nothing I do with it seems to matter, e.g. 

.FS
http://db.lcs.mit.edu/projects/\:cstore/vldb.pdf
.FE

changes the warning, but not the appearance.  

What am I missing?  

--jkl