  We address the question of whether individual nonmagnetic impurities can
induce zero-energy states in time-reversal-invariant topological
superconductors, and define a class of symmetries which guarantee the existence
of such states for a specific value of the impurity strength. These symmetries
allow the definition of a position-space topological Z_2 invariant, which is
related to the standard bulk topological Z_2 invariant. Our general results are
applied to the time-reversal-invariant p-wave phase of the doped
Kitaev-Heisenberg model, where we demonstrate how a lattice of impurities can
drive a topologically trivial system into the nontrivial phase.
