Unable to use pre-match purchased limited-availability skin

I was playing an ARAM, and was traded Fiora. Since I was intending to buy the Harrowing skin for her, I tried to select it in the skin select during the pre-game countdown, but didn't see it. I clicked the "buy skin" button on another skin, then navigated to the Harrowing skin and bought it. However, it wouldn't let me select the skin for that game even after I had purchased it.