How do I fake System.Data.SqlTypes.SqlString?

Im brand new to TypeMock.

When I run this test I get an exception that says:

TypeMock.TypeMockException :
*** Faking behavior on struct System.Data.SqlTypes.SqlString is not supported in the following cases:
1. Faking behavior on more than one struct of the same type.
2. Faking behavior on more than one method on the same struct.

How do I do I correct this for my code?

Here is my Code:

using System;
using System.Data.SqlTypes;
using System.Text;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using TypeMock.ArrangeActAssert;

namespace Arh.Database.SqlFunctions.Tests
{
// ReSharper disable InconsistentNaming

[TestClass]
public class UserDefinedFunctionsTest
{
[Isolated]
[TestMethod]
public void GetExpectedArgParams_ArgNIsNullAndArgNDelimiterIsNull_ReturnsNull()
{
//Arrange – setup for the test
Isolate.Fake.StaticConstructor(typeof(SqlString));

var format = Isolate.Fake.Instance<SqlString>(Members.ReturnRecursiveFakes);
Isolate.WhenCalled(() => format.IsNull).WillReturn(true);

var argN = Isolate.Fake.Instance<SqlString>(Members.ReturnRecursiveFakes);
Isolate.WhenCalled(() => argN.IsNull).WillReturn(true);

//Act – the actual invocation of the method
var userDefinedFunctions = UserDefinedFunctions.FormatString(format, argN, null);

//Assert – verification of pass/fail condition for the test
Assert.IsNull(userDefinedFunctions);
}
}

// ReSharper restore InconsistentNaming
}