XML : Creating a test method to test WriteXML

I have a method WriteXml.

  public void WriteXml(XmlWriter writer)          {              if (this.value == null)              {                  writer.WriteValue(this.value?.ToString() ?? string.Empty);                  return;              }              var type = this.value.GetType();                var memInfo = type.GetMember(this.value.ToString());                if (memInfo.Length > 0)              {                  var attrs = memInfo[0].GetCustomAttributes<XmlEnumAttribute>();                    if (attrs != null && attrs.IsNotEmpty())                  {                      writer.WriteValue(attrs);                        return;                  }              }              writer.WriteValue(this.value.ToString());          }    

When I am testing it with the unit test. i.e.,

  [Theory]          [InlineData(null, "")]          [InlineData(exposureSValue.NA, "N/A")]          [InlineData(exposureSValue.Last15Days, "Last 15 Days")]          public void WriteXmlTest([CanBeNull] exposureSValue? policyCustomEnumValue, [CanBeNull] string writtenValue)          {              // Arrange              var mockXmlWriter = this.Mock.Create<XmlWriter>(Moq.MockBehavior.Loose);              mockXmlWriter.Setup(x => x.WriteValue(writtenValue));                var policyCustomerEnum = new PolicyCustomEnum<exposureSValue>(policyCustomEnumValue);                // Act and Assert              policyCustomerEnum.WriteXml(mockXmlWriter.Object);          }    

While I run this unit test I get an error saying

  The following setups were not matched:  XmlWriter x=>x.WriteValue("N/A");    

Please help me out with this. I also tried changing the parameter in the test method which is string type to exposureSValue type but it still the same.

How to get rid of that error message?