Setting Variables

Have a interesting problem:

If I do something like :

#set( $foo = $bar / 52 )
$foo

where $bar is some Float set in the context (say 52), then I get 
$foo - ie it isn't replaced.

However, if I do this:

#set( $foo = 0 )
#set( $foo = $bar /52 )
$foo

Then the result is:
1

Which is correct. Question is why do I need to set $foo to zero before doing 
the second set for it to work?

Regards,

Peter