The provider user key supplied is invalid. It must be of type System.Guid.

i have the following error when i try to update user information

here 's the code


MembershipUser user = Membership.GetUser(txtSelectedUserName.Text);

if (user != null)
{


if (user.ChangePassword(txtOldPassword.Text, txtNewPassword.Text))
{


if (user.ChangePasswordQuestionAndAnswer(
txtNewPassword.Text, txtNewPasswordQuestion.Text, txtNewPasswordAnswer.Text))

{

lblStatus.Text =
"Success!";
}


else
{

lblStatus.Text =
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