In a 2008 paper, Spekkens improved the traditional notions of non-negativity of Wigner-style quasi-probability distributions and non-contextuality of observations. He showed that the two improved notions are equivalent to each other. Then he proved what he called an even-handed no-go theorem. The paper contains some minor inaccuracies and one false claim, in the proof of the no-go theorem. This claim, early in the proof, is used in an essential way in the rest of the argument. Here we analyze carefully Spekkens’s proof of the no-go theorem, explain the inaccuracies, reduce the task of proving the no-go theorem to the special case of a single qubit, and then prove the special case. This gives us a complete proof of Spekkens’s no-go theorem.