加载SOAP xml时未声明的前缀

Question

我收到客户的文档以使用下面的SOAP请求。我之前从未使用过SOAP，而且我正在使用此问题中列出的解决方案：Client to send SOAP request and received response

然而，当soapEnvelop.LoadXml运行时，我得到：'head' is an undeclared prefix. Line 5, position 39.

这是我从客户端收到的SOAP xml：

<soap:Envelope xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
               xmlns:xsd='http://www.w3.org/2001/XMLSchema'
               xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'>
    <soap:Header>
        <head:CustomCredentials>
            <head:UserName>Username</head:UserName>
            <head:Password>Password</head:Password>
            <head:Source>Code</head:Source>
            <head:CurrentCulture>en-CA</head:CurrentCulture>
        </head:CustomCredentials>
    </soap:Header>
    <soap:Body>
        <WebServiceMethodName xmlns='Web Service Namespace' />                                        
    </soap:Body>
</soap:Envelope>

我不确定该怎么做。从谷歌搜索我相信元素头在命名空间中不存在。