The biggest argument in favor of Constellation Energy stock just became a liability. Electric utility stock Constellation Energy ( CEG -10.85%) tumbled 10% through 10:35 a.m. ET Monday morning despite beating analyst forecasts for third-quarter earnings this morning.

Shares of Constellation Energy Corp. were in danger of a record selloff on Monday and were headed for their lowest prices since before the nuclear-power company signed a 20-year power-purchasing agreement with Microsoft Corp.

Key Takeaways Constellation Energy and other nuclear stocks fell Monday as markets reacted to a new decision from regulators.A partnership between Amazon and Talen was rejected by regulators who said it could raise energy prices for customers in the surrounding area.