Cannot convert from ‘ref int’ to ‘ref int?’

Recently, I kept getting above mentioned error. My code looked like this:

protected void btnUploadSubmit_Click(object sender, EventArgs e)
{
/* here goes code for uploading Excel file and filling DataTable called excelTable*/ BackEndTableAdapters.RegistrantsTableAdapter reg = new BackEndTableAdapters.RegistrantsTableAdapter();

foreach(DataRaow excelRow in excelTable.Rows)
{
int ID=0; // source of evil

string firstName = excelRow[“First Name”].ToString();
string lastName = excelRow[“Last Name”].ToString();
/* … */
reg.AddRegistrant(firstName, lastName,……, ref ID); /*this is the line that generates error; ID is output parameter of stored procedure */
}
/* … */

}

What was wrong with this code? Well, last argument of AddRegistrant function was nullable int (int?) and not int. When I changed line

int ID=0;

into

int? ID=0;

everything went well..

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