diff --git a/lib/types/minimally-encoded-thread-permissions-types.js b/lib/types/minimally-encoded-thread-permissions-types.js
--- a/lib/types/minimally-encoded-thread-permissions-types.js
+++ b/lib/types/minimally-encoded-thread-permissions-types.js
@@ -231,6 +231,9 @@
       minimallyEncodedRawThreadInfo;
     return {
       ...rest,
+      // We removed the `.permissions` field from `MemberInfo`, but persisted
+      // `MemberInfo`s will still have the field in legacy migrations.
+      // $FlowIgnore
       members: members.map(decodeMinimallyEncodedMemberInfo),
       roles: _mapValues(decodeMinimallyEncodedRoleInfo)(roles),
       currentUser: decodeMinimallyEncodedThreadCurrentUserInfo(currentUser),