The other poster is incorrect, that the A is not technically a Bbb, even though it appears to be a 7th it is not.
Ouch! That's me put in my place...
And Walter Piston in his book 'Harmony'. And Schoenberg in his Harmonielehre. And R.O. Morris in 'The Oxford Harmony Volume One'.
Or, to put it another way: what's the seventh chord on the leading note in G minor? It has both sharps and flats in it, by my count.
Perhaps you were referring to a diminished seventh chord on Fsharp? It would be A in that one...
Recording Secrets for the Small Studio
Mixing Secrets for the Small Studio.