While calling a function in webservice its showing an error " Exception Error! Server was unable to process request.Object reference not set to an instance of an object." using php

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PHP

SOAP

WSDL

This is my part of wsdl file

<s:element name="CalculStudents">
    <s:complexType>
        <s:sequence>
            <s:element minOccurs="0" maxOccurs="1" name="doc">
                <s:complexType mixed="true">
                    <s:sequence>
                        <s:any/>
                    </s:sequence>
                </s:complexType>
            </s:element>
        </s:sequence>
    </s:complexType>
</s:element>

using php i connected the wsdl

$client = new SoapClient("some.wsdl");
$params = array("any"=>'');
$result = $client->CalculStudents(array('doc'=>$params));

Now i am getting this error

Exception Error! Server was unable to process
request.Object reference not set to an instance of an object.

Tell me a solution for this

Thanks in advance