empty string parsed as float?

I have a schema which defines an element with type float (or anything other
than string for that matter).
The xml file validated has the element, but it's empty.
So the parser tells me it can't parse the value '' as a float.

When I wrote the schema I thought that if you leave the element empty, it
would pass validation, and if you don't leave it empty, it MUST be a float
to pass validation.
I don't want the element to have the type string or token, I want float. But
I want it to not give an error if the element is empty.
Is this possible? In that case how?


/Gabriel