Superflat generation in 1.13 - custom preset crashes game

When creating a new Superflat world, using a custom preset string. When I click "Use Preset" I see a "saving World" screen for a few seconds, then the game crashes. More Details:

The game crashed whilst mouseclicked event handler
The game crashed whilst mouseclicked event handler
Error: i: Non [a-z0-9/._-] character in path of location: minecraft:gravel,2*minecraft:redstone_ore,minecraft:stone,2*minecraft:gold_ore,minecraft:stone,2*minecraft:diamond_ore,1*minecraft:stone,2*minecraft:iron_ore,1*minecraft:stone,2*minecraft:coal_ore,1*minecraft:stone,2*minecraft:clay,1*minecraft:stone,2*minecraft:sand,5*minecraft:stone,4*minecraft:dirt,minecraft:grass_block

Crash report attached.