On the instability of 3d null singularities

String propagation on a three-dimensional Lorentzian string orbifold with anull singularity has been studied by Horowitz and Steif, and more recently byLiu, Moore and Seiberg. We analyze the target space as a classicalgravitational background. The singularity becomes spacelike when an arbitrarilysmall amount of matter is thrown at the singularity. This can be seen directlyby studying the null singularity as a limit of the M=0, J=0 BTZ black holemetric.Comment: 9 pages, uses harvmac.tex. v2: minor wording changes in introduction, fixed reference typo, new reference