Starting to feel a bit moronic here.
I've included a short worksheet illustrating my problem. I'm trying to graduate from simple calculation to using vectors. As you will see, I have an equation for M for which I can calculate a value easily enough. Units work out OK too.
When I attempt to make a vector (in my world, this lambda equates to a wavelength but that's irrelevant to the problem), I get an error complaining of the units.
Not really sure if that's really the issue here; can anybody else shed some light?