[web2py] How to order drop down

[weheh]

I have Field('freq','integer',requires=IS_IN_SET(FREQS,zero=None)) as
part of a table.

FREQS={
  0:'never',
  900:'15 minutes',
  1800:'30 minutes',
...etc.
}

I want to force the drop down to order ascending by the keys of FREQS.

Anyone know the syntax to do this in the Field declaration?