This is a Mac commandlet crash that is experienced by a few external users.
Not yet reproduced internally. This is the cook command used by an external user (note that the user's system language is Chinese and the cook involves UTF8Output):
/Users/wusiqi/Perforce/wusiqi_wusiqideMacBook-Pro_level-streaming_7442/NewGame.uproject -run=Cook -TargetPlatform=MacNoEditor -fileopenlog -unversioned -numcookerstospawn=8 -abslog=/Users/Shared/EpicGames/UE_4.15/Engine/Programs/AutomationTool/Saved/Cook-2017.04.13-15.17.24.txt -stdout -CrashForUAT -unattended -UTF8Output
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-43828 in the post.