web service error when consuming soap message with parameters

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ASP.NET

jQuery

SOAP

webservice

I have created an asp.net web service which has a method that accepts a string parameter (filter), queries a sql server database and returns the result in XML. I am invoking the web service via soap. When I use a straight soap client (eg soapui) it works fine. HOwever if i call it from a web page using ajax (specifically via jquery), I get the following error:

System.InvalidOperationException: Missing parameter: filter. at System.Web.Services.Protocols.ValueCollectionParameterReader.Read(NameValueCollection collection) at System.Web.Services.Protocols.HtmlFormParameterReader.Read(HttpRequest request) at System.Web.Services.Protocols.HttpServerProtocol.ReadParameters() at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()

I have checked the soap message using tcpmon and the parameter is defined.

If I change the web method so it has no parameters, it also works fine, so it is some issue with passing parameters via soap and ajax.