error on dropdownlist (when edit_profile_page loads then that time data completly filled but on update shows error)

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C#

ASP.NET

SQL-Server

hi,

i have an error on dropdownlist
when i run the page of edit_profile_page
then that time it loads successfully and then that time data also completly filled successfully..
but when i changed the dropdownlist value and click on updatebutton it shows error.

'cmbprofilefor' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

"cmbprofilefor" is my dropdownlist name

i used this line of code for selected item which is stored in database

cmbprofilefor.SelectedValue = dt.Rows[0]["ProfileFor"].ToString().Trim();

this code is successfully work and display value selected in dropdownlist which stored in database.
but when i change the items means when i selected item changed from dropdownlist) and click on save button that time it shows me error.

where is the my mistake ??