Using IsDBNull method of OleDBDataReader throws an error

I am using an OleDbDataReader to retrieve information from an Excel Spreadsheet. When I attempt to test a column for a null value using the IsDBNull method, I get an error "Multiple-step OLE DB operation generated errors. Check each OLE DB status value, if available. No work was done.
The connection for viewing your linked Microsoft Excel worksheet was lost.".
I cannot see what I am doing wrong, having compared my code to lost of examples. Can anyone advise or help me troubleshoot further

OleDbDataReader dr;
string connectionString = @"Provider=Microsoft.Jet.OLEDB.4.0; Data Source=C:\\Uploads\Shelters.xls;Extended Properties=""Excel 8.0;HDR=YES;""";
OleDbConnection conn = new OleDbConnection(connectionString);
conn.Open();
OleDbCommand cmd = new OleDbCommand("Select route from [AA$]", conn);
dr = cmd.ExecuteReader();
string route;
while (dr.Read())
{
       if (dr.IsDBNull(0)) //This line produces the error
       {
          route = "";
       }
       else
       {
          route = dr.GetString(0);
       }
}
dr.Close();
conn.Close();

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