The UK stock market's home to some of the most generous dividend shares on the planet, making it remarkably easy to unlock a chunky second income almost overnight. One standout example currently pays enough to turn a brand new £20,000 ISA into roughly £1,436 of instant passive income. Meet the REIT behind this yieldSupermarket Income REIT (LSE:SUPR) buys supermarket buildings and leases them back to major grocery chains like Tesco and Sainsbury's on long-term contracts. And yet at the same time, net rental income actually fell by 2% to £57m. Where things get complicatedSupermarket Income REIT's dividend coverage issue doesn't stem from a weakened rental income stream.