Syntax highlighting bug: Attribute targeting Enum value

I take it this needs to be declared something like this:

[AttributeUsage(AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
public class MyCustomAttribute : Attribute
{
    //Some properties...
}

public enum MyEnum : int
{
    [MyCustom]
    One = 1,
    [MyCustom]
    Two = 2,
    [MyCustom]
    Three = 3,
}

However, syntax highlighting is claiming the following:

With red underlining, "Attribute is not valid on this declaration type. It is valid on 'Enum' declarations only.

However, to the compiler, this builds interestingly enough.

I am targeting a .NET 4.5 that I know of, x64 bit.

Is this some obscure C# thing? Or a Resharper bug?

Thank you...