cursor.executemany() float error

[progman]

Data Struct:
from (string), to (string), rate (float)

when i run this:

cursor.executemany('insert into promo (`From`,`To`, `RATE`) \
values (%s,%s,%f)',  [ ('AA','BB',10.2), ('CC','DD',10.3) ] )

i got this error:
TypeError: float argument required

i checked, 10.2 & 10.3 , there are at the right loc.
what went wrong??