[web2py] Why there is a blank option in drop-down and how to avoid it?

[at]

Hi,

When I create a SQLFORM for a table containing following field, it creates 
a drop-down listing all specified fields and *additionally a blank option*. 
How can we avoid it?

Field('code', default='F3',
                       requires=IS_IN_SET(['U1', 'F3', 'T9', 'K3', 'S2', 
'M1', 'Z5'])),

Thanks

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