WebGL libil2cpp error since b17

Hi everyone, we’ve been getting an error (below) in our WebGL builds since b17. b18 also has this problem. If we revert to b15 our game loads fine.
The error isn’t exactly the most helpful thing, but I’m hoping someone can make sense of it? Thanks!

uncaught exception: Assertion failed: !field->type->byref, at: C:\Program Files\Unity 5.4.0b18\Editor\Data\il2cpp\libil2cpp\vm\Class.cpp,1767,GetBitmapNoInit at jsStackTrace@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:1:22715
stackTrace@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:1:22898
___assert_fail@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:1:261847
d8r@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:34:3108
a2t@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:34:942607
invoke_viiii@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:1:386935
aMt@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:34:513264
R7r@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:10:1147094
L$t@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:34:926552
invoke_vii@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:1:362469
qrr@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:10:592797
ztr@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:10:646634
Jur@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:10:686713
Q4a@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:22:818526
w1b@blob:https://redshift.clipwiregames.com/8ed3c79c-ac0d-408d-9310-cbe6d98454c1:17:1074900
callMain@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:41:57785
doRun@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:41:58612
run/<@blob:https://redshift.clipwiregames.com/74005559-36b0-40f7-a9b4-58448fa7d994:41:58780

Could you please send a bug report with a repro project attached and ping the case number here, please?