Best exception for an invalid generic type argument

I'm currently writing some code for UnconstrainedMelody which has generic methods to do with enums.

Now, I have a static class with a bunch of methods which are only meant to be used with "flags" enums. I can't add this as a constraint... so it's possible that they'll be called with other enum types too. In that case I'd like to throw an exception, but I'm not sure which one to throw.

Just to make this concrete, if I have something like this:

// Returns a value with all bits set by any values
public static T GetBitMask<T>() where T : struct, IEnumConstraint
{
    if (!IsFlags<T>()) // This method doesn't throw
    {
        throw new ???
    }
    // Normal work here
}

What's the best exception to throw? ArgumentException sounds logical, but it's a type argument rather than a normal argument, which could easily confuse things. Should I introduce my own TypeArgumentException class? Use InvalidOperationException? NotSupportedException? Anything else?

I'd rather not create my own exception for this unless it's clearly the right thing to do.