Unable to access .net webservice in php using soap

<pre lang="c#">[WebMethod]
        public string EncryptText1(string plaintext)
        {

            ASCIIEncoding textConverter = new ASCIIEncoding();
            byte[] key = textConverter.GetBytes("2a1c907916add59edffb3a4b");
            byte[] IV = textConverter.GetBytes("00000000");
            byte[] clearData = Encoding.ASCII.GetBytes(plaintext);
            byte[] cipherData = EncryptText(clearData, key, IV);
            return Convert.ToBase64String(cipherData);
        }
    [WebMethod]
    public byte[] EncryptText(byte[] clearData, byte[] Key, byte[] IV)
    {

        MemoryStream ms = new MemoryStream();
        TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
        tdes.Mode = CipherMode.ECB;
        tdes.Padding = PaddingMode.PKCS7;
        ICryptoTransform alg = tdes.CreateEncryptor(Key, IV);
        CryptoStream cs = new CryptoStream(ms, alg, CryptoStreamMode.Write);
        cs.Write(clearData, 0, clearData.Length);
        cs.FlushFinalBlock();
        cs.Close();
        byte[] encryptedData = ms.ToArray();
        return encryptedData;
    }

I have the following code to access the web service in php 

<?php
$enc_wsdl = "http://172.18.0.75/EncryptionWS/EncryptionWS.asmx?wsdl";
$enc_client = new SoapClient($enc_wsdl);
$finalstring = $enc_client->EncryptText1("SomeUserName");
print_r($finalstring);
?>

but I am getting the following error: 

Fatal error: Uncaught SoapFault exception: [soap:Server] Server was unable to process request. ---> String reference not set to an instance of a String. Parameter name: s in C:\xampp\htdocs\dotnetwebservice\index.php:4 Stack trace: #0 C:\xampp\htdocs\dotnetwebservice\index.php(4): SoapClient->__call('EncryptText1', Array) #1 C:\xampp\htdocs\dotnetwebservice\index.php(4): SoapClient->EncryptText1('SomeUserName') #2 {main} thrown in C:\xampp\htdocs\dotnetwebservice\index.php on line 4