Hi Sam,
The error is because system could not be able to match selected value with the value list provided for drop down list.
i.e. drop down list is having values like this
00001 text 1
00002 text 2
but when you select a value, due to screen conversion, the key 1 or 2 is passed instead of 00001 or 00002.
Check out the values and try to format the list of drop down while populating it.
Hope this helps you.
Regards,
Rama