[web2py] admin complains at encoder

[Carl]

in my db.py I have

import datetime
rom time import mktime, localtime
timestamp = SQLCustomType(type='datetime',
                      native='NUMERIC(16,6)',
                      encoder=(lambda x: str(mktime(x.timetuple()) +
x.microsecond/1000000.0 if x else None)),
                      decoder=(lambda x: datetime.datetime(*
(list(localtime(int(x))[:6])+[int(round(x%1,6)*1000000)]) ) if x else
None))

I then use timestamp for various fields.

In /admin when attempt to update a row this is kicked out:
File "E:/projects/workspace/TestEnvoy/web2py/applications/init/models/
db.py", line 39, in <lambda>
    encoder=(lambda x: str(mktime(x.timetuple()) + x.microsecond/
1000000.0 if x else None)),
AttributeError: 'str' object has no attribute 'timetuple'

Any idea where I've gone a stray?