soap exception

Hi All,
I'm new user of SoapPro. After imported a wsdl and substituted the input fields from a datasource, I receive a soap exception invoking the call:
<soap:Fault>
<faultcode>soap:Client</faultcode>
<faultstring>System.Web.Services.Protocols.SoapException: Server was unable to read request. ---> System.InvalidOperationException: There is an error in XML document (7, 50). ---> System.FormatException: Input string was not in a correct format.
at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer&amp; number, NumberFormatInfo info, Boolean parseDecimal)
at System.Number.ParseInt64(String value, NumberStyles options, NumberFormatInfo numfmt)
at System.Xml.XmlConvert.ToInt64(String s)
at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReader1.Read11_GetSSVBasicDataList()
at Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer16.Deserialize(XmlSerializationReader reader)
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
--- End of inner exception stack trace ---
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle)
at System.Web.Services.Protocols.SoapServerProtocol.ReadParameters()
--- End of inner exception stack trace ---
at System.Web.Services.Protocols.SoapServerProtocol.ReadParameters()
at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()</faultstring>
<detail/>
</soap:Fault>

can anyone help me to understand the problem?
Thank in advance
Salvo2011