The Federal Reserve made a lot of money off its bond holdings last year, but in these numbers lies a potential problem for the future.

The Fed took in a record $91 billion. After expenses it sent $88.9 billion to the Treasury, making the Fed a notable force in cutting deficits. But with around a $3 trillion balance sheet, the Fed paid $4 billion in interest on excess reserves.

Central bankers do this to keep the massive amount of bank reserves on the Fed’s books, preventing it from flooding into the economy and starting an inflation surge. At some point the Fed will tighten rates and as part of doing that it will also have to raise the rate it pays on excess reserves.