"Cannot use WhenCalled without a complementing behavior

I have two very similar tests. One of them passes and one fails, throwing the exception "Cannot use WhenCalled without a complementing behavior statement." Here is a minimal repro:

namespace TypeMockReproTest
{
    using System.Xml.Linq;
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using TypeMock.ArrangeActAssert;

    [TestClass, Isolated]
    public class TypemockReproTest
    {
        [TestMethod]
        public void TestOne()
        {
            Isolate.Fake.StaticMethods(typeof(XElement), Members.CallOriginal);
            Isolate.WhenCalled(() => XElement.Load((string)null)).WillReturn(XElement.Parse(string.Empty));
        }

        [TestMethod]
        public void TestTwo()
        {
            Isolate.Fake.StaticMethods(typeof(XElement), Members.CallOriginal);
            Isolate.WhenCalled(() => XElement.Load((string)null)).WillReturn(XElement.Parse("<foo>"));
        }
    }
}

Test One throws the exception and Test Two passes. Any ideas?

Thanks,
Larry