  We use string field theory to fix the normalization of the D-instanton
corrections to the superpotential involving the moduli fields of type II string
theory compactified on an orientifold of a Calabi-Yau threefold in the absence
of fluxes. We focus on $O(1)$ instantons whose only zero modes are the four
bosonic modes associated with translation of the instanton in non-compact
directions and a pair of fermionic zero modes associated with the two
supercharges broken by the instanton. We work with a generic superconformal
field theory and express our answer in terms of the spectrum of open strings on
the instanton. We analyse the contribution of multi-instantons of this kind to
the superpotential and argue that it vanishes when background fluxes are
absent.
