I've gotten that as well, but try and use it. If it doesn't work, just switch it back to default.
Some of the source names for textures may have changed, but just rename them to the new versions, and those will show. Anything that isn't in the pack will just show up as the vanilla texture instead for me.
I did a 1.13 quick fix so I could use it in snapshots (no new textures, but everything renamed/reorganized as needed for the 1.12.2 textures to show up in 1.13). I believe xMrVizzy is working on updating for 1.13 as well. Check his Patreon page, https://www.patreon.com/xmrvizzy