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
@@ -197,6 +197,9 @@
     };
   } else {
     const { members, roles, currentUser, ...rest } = rawThreadInfo;
+    // We removed the `.permissions` field from `MemberInfo`, but persisted
+    // `MemberInfo`s will still have the field in legacy migrations.
+    // $FlowIgnore
     return {
       ...rest,
       minimallyEncoded: true,