Stability conditions for a class of interconnected systems modeled by linear abstract evolution equations and a memoryless nonlinearity are derived. These conditions are stated in terms of the passivity of each of the subsystems and can be considered as a partial generalization of the hyperstability theorem. A Lyapunov function approach is used in the proof without requiring the positive definiteness of the Lyapunov function. Application to the robustness analysis of the infinite-dimensional linear quadratic regulator is also discussed.