Potency, or that which can be, is the subject of both form, or act, and privation.
Evil is essentially the privation of some perfection belonging to a nature.
Therefore, potency is the proper subject of evil.
That which lacks potency, then, cannot be the subject of any evil.
God is Pure Act, meaning he does not contain any potency within Himself.
Therefore, God cannot be the subject of any evil.