2001 Encarta College Dictionary.
Correct usage - Ain't is one of the most informal verb contractions in English, and its use in formal contexts may be criticized because it is associated with careless speech. It is, however, accepted in folk and popular song lyrics, show titles, direct quotations, and fictional dialogue. Otherwise ain't is best avoided, except as a deliberate rhetorical device and in allusive expressions such as You ain't seen nothing yet.
So it seems as if Frank was grammatically correct.
Correct usage - Ain't is one of the most informal verb contractions in English, and its use in formal contexts may be criticized because it is associated with careless speech. It is, however, accepted in folk and popular song lyrics, show titles, direct quotations, and fictional dialogue. Otherwise ain't is best avoided, except as a deliberate rhetorical device and in allusive expressions such as You ain't seen nothing yet.
So it seems as if Frank was grammatically correct.