The problem is the 10 percent threshold.
Berkshire filed an application with the Fed this month, first reported by Bloomberg.
Berkshire already has a 9.96 percent stake and assured the regulator that it will passively invest in the bank and will not try to force a change in strategy or corporate structure, according to Reuters.
The Federal Reserve only allows investors to take double-digit stakes as long as the buyer won't try to force a controlling influence.
Berkshire has large positions in financial services, including American Express, Wells Fargo and JPMorgan Chase & Co.
Berkshire wanted to do the same thing with share of Wells Fargo in 2007, but withdrew after the bank was beset by a scandal.