what's wrong with this loop syntax?

["Michael Newberry" <mnewberry@...>]

Pardon me for asking a simple question, but why can't I use this syntax?

   while (str = S:Next()) ~= nil do

or this syntax:

   while (str = S:Next()) and str ~= nil do

where S exists and S:Next() returns a string or nil.

I get this message:    `)' expected near `='

Thanks,

Michael