diff --git a/keyserver/src/updaters/thread-permission-updaters.js b/keyserver/src/updaters/thread-permission-updaters.js
--- a/keyserver/src/updaters/thread-permission-updaters.js
+++ b/keyserver/src/updaters/thread-permission-updaters.js
@@ -229,7 +229,7 @@
     const ancestorMembership = ancestorMembershipInfo.get(userID);
     if (ancestorMembership) {
       permissionsFromParent = ancestorMembership.permissionsFromParent;
-      memberOfContainingThread = ancestorMembership.memberOfContainingThread;
+      memberOfContainingThread = !!ancestorMembership.memberOfContainingThread;
     }
     if (!hasContainingThreadID) {
       memberOfContainingThread = true;