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.