This paper examines the relation between the structure of a compact MOSFET model and its ability to model harmonic distortion. It is found that non-singular behavior at zero drain bias is essential for qualitatively correct simulations of the third harmonic power dependence. Specifically, nonlinear distortion analysis requires that the Gummel symmetry condition be satisfied by the compact model. A simple procedure to enforce the Gummel symmetry without increasing the complexity of the model is incorporated in an advanced surface-potential-based MOSFET model to enable correct harmonic distortion modeling.