Strange behavior of atan2

Dear all,

I've got a problem with the function atan2. For a couple of coordinates 
x and y,
This function returns the angle between the vector of coordinates (x, y) 
and the
abscissa axis, i.e. it is the same as atan(y/x) (as indicated on the 
help page).
If we consider the vector with coordinates x = 0 and  y = 0, we have
the following result:

 > atan(0/0)
[1] NaN

This is expected. However:

 > atan2(0,0)
[1] 0

Instead of a missing value, the function atan2 returns an angle equal to 
0 radians.
I've searched through the help pages, the FAQ and the forum, but I 
did'nt find
any explanation to this result. Does anyone know if this behavior is 
expected, and
why ?
Thank you for any clues.
Regards,

Cl?ment Calenge

Cl?ment CALENGE
LBBE - UMR CNRS 5558 - Universit? 
Claude Bernard Lyon 1 - FRANCE
tel. (+33) 04.72.43.27.57
fax. (+33) 04.72.43.13.88