A spectral set in R n is a set of finite Lebesgue measure such that L 2 ( ) has an orthogonal basis of exponentials {e 2πi λ,x : λ ∈ } restricted to . Any such set is called a spectrum for . It is conjectured that every spectral set tiles R n by translations. A tiling set T of translations has a universal spectrum if every set that tiles R n by T is a spectral set with spectrum . Recently Lagarias and Wang showed that many periodic tiling sets T have universal spectra. Their proofs used properties of factorizations of abelian groups, and were valid for all groups for which a strong form of a conjecture of Tijdeman is valid. However, Tijdeman's original conjecture is not true in general, as follows from a construction of Szabó [17], and here we give a counterexample to Tijdeman's conjecture for the cyclic group of order 900. This article formulates a new sufficient condition for a periodic tiling set to have a universal spectrum, and applies it to show that the tiling sets in the given counterexample do possess universal spectra.