Full Idea
Kant thinks that if virtue becomes a stable disposition of the person, then it turns into a rigid mechanical habit, with respect to which the person is passive, and thus not fully morally accountable.
Gist of Idea
Kant thinks virtue becomes passive, and hence morally unaccountable
Source
report of Immanuel Kant (Groundwork of the Metaphysic of Morals [1785]) by Julia Annas - The Morality of Happiness 2.1
Book Reference
Annas,Julia: 'The Morality of Happiness' [OUP 1995], p.52