checkbox validation prob

hello
i have a checkbox prob
i would like to give a custom message when a cb is not checked
this fails:
tos = validators.NotEmpty(messages={'empty': 'I am empty'})
and the message i get is " Missing value"
from what i read this is expected as a default from Schema
when a field does not exists
when i try to give a default  val as '' 
 tos = validators.NotEmpty(if_missing='' , messages={'empty': 'I am empty'})
i dont get much succes and it is no more considered as empty
so what can i do to get my message for an unchecked cb
thanks


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