Warren Buffett’s Berkshire Hathaway conglomerate has lost its top-tier credit rating by Standard & Poor’s, CNNMoney reported.

The move, which cut Berkshire’s rating to “AA+,” down one notch from “AAA,” comes as it readies to complete its purchase of railroad operator Burlington Northern Santa Fe Corp. (BNSF).

The downgrade fell on the same day Berkshire filed to sell $8 billion worth of bonds to help finance its $26 billion acquisition of BNSF.

“We believe that the railroad acquisition will reduce what historically has been extremely strong capital adequacy and liquidity, and that investment risk with sizable concentrations remains very high,” S&P said in a statement.

The Omaha-based Fortune 500 company was placed on credit watch in November, following Berkshire’s announcement that it would acquire BNSF.

“Albeit weakened, we view the company’s liquidity position and balance sheet as still very strong,” S&P said.

“However, we see meaningful exposure to adverse development of reserves held for long-term insurance liabilities, and uncertainty remains regarding management succession.”