Iris 1.8.0b1 crashes the jvm

I get a jvm crash soon after joining a world (running vanillaAA shader)
EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x0000020db526d815, pid=10728, tid=13144
J 39253 c2 net.minecraft.class_287.fillExtendedData(I)V (412 bytes) @ 0x0000020db526d815 [0x0000020db526d740+0x00000000000000d5]
This is not a pure Iris instance, it has other mods (notably EMF listed in the stack traces, and betterclouds)