  D-instanton world-volume theory has open string zero modes describing
collective coordinates of the instanton. The usual perturbative amplitudes in
the D-instanton background suffer from infra-red divergences due to the
presence of these zero modes, and the usual approach of analytic continuation
in momenta does not work since all open string states on a D-instanton carry
strictly zero momentum. String field theory is well-suited for tackling these
issues. However we find a new subtlety due to the existence of additional zero
modes in the ghost sector. This causes a breakdown of the Siegel gauge, but a
different gauge fixing consistent with the BV formalism renders the
perturbation theory finite and unambiguous. At each order, this produces extra
contribution to the amplitude besides what is obtained from integration over
the moduli space of Riemann surfaces.
