Checkboxlist selected values is invalid

 i m getting error when select multiple values in checkboxlist(checboxlist

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

)

string cbl = "";
protected void cblDays_SelectedIndexChanged(object sender, EventArgs e)
{

foreach (ListItem li in cblDays.Items)
{
if (li.Selected == true)
{
cblDays.SelectedValue += li.Text;
}
}
}