scalar and untrapable error

[frankie]

I have a problem
When I execute scalar to retreive some data,if it is not
found I get an error which I cannot trap
When nothing is found, response is "" (length=0), but it comes from catch.
Also, if I don't include conn.close in catch,conenction remains open

Point is that I have read on several places that using scalar is the fastest
method to get single data and I use it often (in one report more than 1000
times).
When I use DataReader,it is much slower. So if anybody can help me how to
trap when nothing is found
public string getEntry(string cs, string sql)
{
OleDbConnection conn = new OleDbConnection(cs);
string response="";
try
{

conn.Open();
OleDbCommand idCMD = new OleDbCommand(sql, conn);

response = (string)idCMD.ExecuteScalar();

if (response.Length<1)
return "failed";

return response;
}

catch
{
if (conn !=null) {conn.Close();}
return "failed";
}
finally
{
if (conn !=null) {conn.Close();}
}
}