using xsi:type="xsd:string"

In the head of my XFORM document, I have an instance whose type is set 
to "xsd:string". In the body, there is a corresponding <input> element.

At run-time (using FormsPlayer), the text box appears with the alert 
message displayed as if the input is not well-formed and unless the user 
enters a non-null string into the text box, the form will not submit. I 
found at run-time that entering a single space was sufficient to make it 
happy (the alert disappears and the form can be submitted). However, if 
I try to provide a single space as the default value in the instance, it 
behaves as if no default value was provided (displays the alert and will 
not submit).

Is there anything that mandates (in XSD or XFORMS) that xsd:string must 
be non-null? Or is this just an interpretation that FormsPlayer is 
making?

Kerry