NYSE MKT is suffering an outage in trading certain Nasdaq stocks.

Shortly after the open Tuesday, NYSE MKT formerly known as the American Stock Exchange, announced a problem with processing quotes in Nasdaq UTP symbols on the NYSE MKT cash equities market.

By late morning, the exchange said that it was recovering one matching engine that was affecting order processing and trade execution in 150 Nasdaq symbols. During the recovery process, the market didn’t publish any quotes in the affected symbols.

By midday, NYSE MKT declared that it would not resume trading in the 150 affected symbols for Tuesday. It did not give any indication when trading may resume in those securities.

The latest trading outage comes amid an ongoing debate about market structure in the U.S., where some are worried that outages such as these are indicative of excessive complexity and fragility in the U.S. equities markets.