A new mechanism for causing naked singularities is found in an effective superstring theory. We investigate the gravitational collapse in a spherically symmetric Einstein-Maxwell-dilaton system in the presence of a pure cosmological-constant “potential,” where the system has no static black hole solution. We show that once gravitational collapse occurs in the system, naked singularities necessarily appear in the sense that the field equations break down in the domain of outer communications. This suggests that in generalized theories of gravity, the nonminimally coupled fields generically cause naked singularities in the process of gravitational collapse if the system has no static or stationary black-hole solution. © 1998 The American Physical Society.