R1:3211238a950c

Fix a bug found by vrana in audit

Summary: seen https://secure.phabricator.com/rPf5e71eb88920d4afb5590b0682eda66f09b2c024

Test Plan: i believe in vrana.

Reviewers: vrana

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D3588