Couldn't find selectedvalue

I'm having problem with GridView again.
I don't know what'd happened.
It worked on last Friday, I was really happy that it finally worked.
But, today when I tested it again, it failed.

Scenario:
I added an edit function to the GridView, and bind the values of the fields to the dropdownlists.
Problem happens after I clicked on the "edit" button on the selected row.
I populated the dropdownlist with a DataSet that was returned from my own function.
DataSource='<#% GetCategories() %>'
I've checked the DataSource passed in correctly, with all the items I want to list out.
But, error returned saying the dropdownlist selected value was not found in the item list.
How could this happen?
The dropdownlist is bond to the correct field.
The weird thing is, I didn't change any codes here.
I'm a bit frustrated now, there are many more work for me to do and problems to solve.
Yet, something was done and now it's undone.

Well, I know frustration won't help to solve the problem.
I will take a break here, and do something else first.
I'll finally find out how to solve this later.
Sometimes, a simple thing but costs lots of effort & thinking.