The bond market is doing something it hasn’t done since 2007 — and could be signaling a recession

The yield curve is flatter than it has ever been since before the financial crisis.

If it gets any worse, there'll be lots of talk of a possible recession, says Craig Johnson, chief market technician at Piper Jaffray. He told CNBC's "Trading Nation" on Friday why this could be a warning sign for markets.

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