This reading is an exception, but there is another way you can say it that uses the jukugo (on'yomi) readings instead! But to help you remember the more common, exception, here's a mnemonic:

If you go to an old shop you can get what's called a shiny sale (しにせ). A shiny sale is if you can find something in the shop that's still new and shiny it's on sale! It's hard when you're at an old shop though. And that's the challenge!