SoapUI isn't serializing nullable int

I have datacontract with datamember of type nullable int .

 [DataMember(Order = 5,EmitDefaultValue = false)]
        public int? Zip { get; set; }

When I leave the Zip value empty and submit the request trough SoapUI I'm getting this error :

<InnerException i:nil="true"/><Message>Input string was not in a correct format.</Message><StackTrace>at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer&amp; number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at System.Xml.XmlConvert.ToInt32(String s)
   at System.Xml.XmlConverter.ToInt32(String value)</StackTrace><Type>System.FormatException</Type></InnerException><Message>The value '' cannot be parsed as the type 'Int32'.</Message><StackTrace>at System.Xml.XmlConverter.ToInt32(String value)
   at System.Xml.XmlDictionaryReader.ReadElementContentAsInt()
   at System.Runtime.Serialization.XmlReaderDelegator.ReadElementContentAsInt()
   at ReadAddressSearchRequestFromXml(XmlReaderDelegator , XmlObjectSerializerReadContext , XmlDictionaryString[] , XmlDictionaryString[] )
   at System.Runtime.Serialization.ClassDataContract.ReadXmlValue(XmlReaderDelegator xmlReader, XmlObjectSerializerReadContext context)
   at System.Runtime.Serialization.XmlObjectSerializerReadContext.ReadDataContractValue(DataContract dataContract, XmlReaderDelegator reader)
   at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator reader, String name, String ns, Type declaredType, DataContract&amp; dataContract)
   at System.Runtime.Serialization.XmlObjectSerializerReadContext.InternalDeserialize(XmlReaderDelegator xmlReader, Type declaredType, DataContract dataContract, String name, String ns)
   at System.Runtime.Serialization.DataContractSerializer.InternalReadObject(XmlReaderDelegator xmlReader, Boolean verifyObjectName, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.XmlObjectSerializer.ReadObjectHandleExceptions(XmlReaderDelegator reader, Boolean verifyObjectName, DataContractResolver dataContractResolver)</StackTrace><Type>System.Xml.XmlException</Type></InnerException>

When I run this through .net client , the request is serialized correctly and I'm getting good response.

Any ideas why I have this problem in SoapUI?

Thanks.