Very strange date problem for the month may

I use the calendar_date_select plugin
which is working fine, but the strange thing is:
if i debug my application, i can see that the date is received by the
controller in the format ‘july 10, 2007’

somewhere this gets translated (not by my application) in a format which
mysql understands, because i can save this record correct and in the log
i can see the submit query is submitted with the date field “2007-07-10”

however, if i select a day in may, this is not working anymore.
in the log i see the date: ‘may 10, 2007’ as the string which gets
submitted, but it doesn’t pass field validation, and if i remove field
validatio for this field, i get:

You have a nil object when you didn’t expect it!
The error occurred while evaluating nil.strftime

is this a bug somewhere?

regards, remco