Investors’ preference for the most- liquid corporate debt is running higher than any time since the credit crisis, a signal they’re preparing for the four-year rally to end.

The expense incurred by credit traders to complete bond transactions was the lowest last year relative to costs implied by the market’s average bid-ask spread since 2009, according to Barclays (BCS).

The shift, a sign that buyers are favoring securities that are easiest to trade, has helped financial bonds beat industrial debt by the biggest margin on record, Bank of America Merrill Lynch (BAC) index data show.