dropdownlist in gridview get error

Hi,
I follow step by step msdn guide (in french)   http://msdn.microsoft.com/fr-fr/library/ms178294.aspx
(you can find in msdn in english, or i made Yahoo translate in the link below)

http://fr.babelfish.yahoo.com/translate_url?doit=done&tt=url&intl=1&fr=bf-home&trurl=http%3A%2F%2Fmsdn.microsoft.com%2Ffr-fr%2Flibrary%2Fms178294.aspx&lp=fr_en&btnTrUrl=Traduire
I got the error " DropDownList1' has a SelectedValue which is not valid, because it doesn't exist in element list. Parameter name: value" when I click on Modify option in the gridview. Or my list has no parameter which name is value.
What did I do wrong? Please help!

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