FluentValidation - Value must be assignable; Parameter name: left

I am trying to mock my fluentValidation validators in unit tests. For some reason i have a couple that throw this error?

Here is my entity (EF6)

public class Content : AuditableEntity
{
    public long PageId { get; set; }

    public virtual Page Page { get; set; }

    public short Position { get; set; }

    public short Width { get; set; }

    public Content() 
    {
        Position = 1;
    }

}

Here is my validator

public class ContentValidator : AbstractValidator<Content>
{
    public ContentValidator()
    {
        RuleFor(e => e.Page).NotNull().WithMessage("You must select a page for this content").When(e => e.PageId == 0);

        RuleFor(e => e.Position).GreaterThan((short)0).LessThan((short)256).WithMessage("Position must be between 1 and 255");

        RuleFor(e => e.Width).GreaterThan((short)0).LessThan((short)13).WithMessage("Width must be between 1 and 12");
    }
}

Here are my mock tests (that fail)

private ContentValidator _validator;

[TestInitialize]
public void TestInit()
{
    _validator = new ContentValidator();
}
[TestMethod]
public void ContentValidator_ShouldHaveErrorWhenPositionLessThanOne()
{
    _validator.ShouldHaveValidationErrorFor(e => e.Position, 0);
}

[TestMethod]
public void ContentValidator_ShouldHaveErrorWhenPositionGreaterThanTwoHundredAndFiftyFive()
{
    _validator.ShouldHaveValidationErrorFor(e => e.Position, 256);
}

[TestMethod]
public void ContentValidator_ShouldHaveErrorWhenWidthLessThanOne()
{
    _validator.ShouldHaveValidationErrorFor(e => e.Width, 0);
}

[TestMethod]
public void ContentValidator_ShouldHaveErrorWhenWidthGreaterThanTwelve()
{
    _validator.ShouldHaveValidationErrorFor(e => e.Width, 13);
}

For some reason these tests throw an error. Here is the error from one of the tests, the rest are all the same, just different method

Test Name:  ContentValidator_ShouldHaveErrorWhenPositionGreaterThanTwoHundredAndFiftyFive
Test FullName:  Tests.TestContentValidator.ContentValidator_ShouldHaveErrorWhenPositionGreaterThanTwoHundredAndFiftyFive
Test Source:    e:\Sample Projects\Tests\TestContentValidator.cs : line 75
Test Outcome:   Failed
Test Duration:  0:00:00.001921

Result Message: 
Test method TestContentValidator.ContentValidator_ShouldHaveErrorWhenPositionGreaterThanTwoHundredAndFiftyFive threw exception: 
System.ArgumentException: Expression must be writeable
Parameter name: left
Result StackTrace:  
at System.Linq.Expressions.Expression.RequiresCanWrite(Expression expression, String paramName)
   at System.Linq.Expressions.Expression.Assign(Expression left, Expression right)
   at FluentValidation.MemberAccessor`2.CreateSetExpression(Expression`1 getExpression) in c:\Projects\FluentValidation\src\FluentValidation\MemberAccessor.cs:line 29
   at FluentValidation.MemberAccessor`2..ctor(Expression`1 getExpression) in c:\Projects\FluentValidation\src\FluentValidation\MemberAccessor.cs:line 23
   at FluentValidation.MemberAccessor`2.op_Implicit(Expression`1 this) in c:\Projects\FluentValidation\src\FluentValidation\MemberAccessor.cs:line 66
   at FluentValidation.TestHelper.ValidatorTester`2..ctor(Expression`1 expression, IValidator`1 validator, TValue value, String ruleSet) in c:\Projects\FluentValidation\src\FluentValidation\TestHelper\ValidatorTester.cs:line 33
   at FluentValidation.TestHelper.ValidationTestExtension.ShouldHaveValidationErrorFor[T,TValue](IValidator`1 validator, Expression`1 expression, TValue value, String ruleSet) in c:\Projects\FluentValidation\src\FluentValidation\TestHelper\ValidatorTestExtensions.cs:line 29
   at Tests.TestContentValidator.ContentValidator_ShouldHaveErrorWhenPositionGreaterThanTwoHundredAndFiftyFive() in e:\Sample Projects\Tests\TestContentValidator.cs:line 76