NullReferenceException w/ authorize.net

getting a NullReferenceException error on this line: myWriter.Close();

in this code:

public static string CreditCardProcessorCaller(string[] arrCCData) {

string strResult = "";
string strPost = "x_login=" + "someuser" +
"&x_password=" + "somepassword" +
"&x_version=3.1" +
"&x_delim_data=TRUE" +
"&x_delim_char=|" +
"&x_method=CC" +
"&x_recurring_billing=NO" +
"&x_type=AUTH_CAPTURE" +
"&x_amount=5.00" +
"&x_card_num=" + arrCCData[0] +
"&x_exp_date=" + arrCCData[1] +
"&x_card_code=" + arrCCData[2] +
"&x_cust_id=" + arrCCData[3] +
"&x_customer_ip=" + arrCCData[4] +
"&x_first_name=" + arrCCData[5] +
"&x_last_name=" + arrCCData[6] +
"&x_address=" + arrCCData[7] +
"&x_city=" + arrCCData[8] +
"&x_state=" + arrCCData[9] +
"&x_zip=" + arrCCData[10];

StreamWriter myWriter = null;
HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create("https://security.authorize.net/gateway/transact.dll");
objRequest.Method = "POST";
objRequest.ContentLength = strPost.Length;
objRequest.ContentType = "application/x-www-form-urlencoded";

try
{
myWriter = new StreamWriter(objRequest.GetRequestStream());
myWriter.Write(strPost);
}
catch (Exception strException)
{
return strException.ToString();
}
finally
{
myWriter.Close();
}

HttpWebResponse objResponse = (HttpWebResponse)objRequest.GetResponse();
using (StreamReader sr = new StreamReader(objResponse.GetResponseStream()))
{
strResult = sr.ReadToEnd();
sr.Close();
}

return strResult;
}