I just set up a GridView control on a new page, got the <columns> set up the way I need, and when I tried to "Edit" and "Update", I got the error message above.
I couldn't tell which field was causing the error, so I cut them all out of the control, except for a few text fields that already had values, and started adding them back one at a time to see which was the problem field.
The fields I added back were of assorted data types (text, date, currency, yes/no), and the page was working fine after each restored field.
But finally, when I added back a field that was just a straight number, I got the error.
And when I took that column back out, I STILL got the error!
Any ideas what the problem is?