A broad set of bond exchange-traded funds are trading out of sync with their underlying assets, testing investors’ faith in a fast-growing part of the investment world.

Bond ETFs of BlackRock, Vanguard Group and others traded at historic discounts to the net asset value of their underlying bonds in recent days. BlackRock’s iShares iBoxx USD Investment Grade Corporate Bond ETF closed down at a discount of over 5% late last week, a record since 2008. The $28bn bond fund, with the ticker LQD, typically trades within a fraction of a percent of the bonds it is designed to track. It closed at a discount of over 1.5% every day of the past week.