diff --git a/keyserver/src/creators/thread-creator.js b/keyserver/src/creators/thread-creator.js
--- a/keyserver/src/creators/thread-creator.js
+++ b/keyserver/src/creators/thread-creator.js
@@ -51,6 +51,7 @@
   recalculateThreadPermissions,
   commitMembershipChangeset,
   getChangesetCommitResultForExistingThread,
+  type MembershipChangeset,
 } from '../updaters/thread-permission-updaters.js';
 import { joinThread } from '../updaters/thread-updaters.js';
 import RelationshipChangeset from '../utils/relationship-changeset.js';
@@ -328,16 +329,12 @@
     await dbQuery(query);
   }
 
-  let initialMemberPromise;
-  if (initialMemberIDs) {
-    initialMemberPromise = changeRole(id, initialMemberIDs, null, {
-      setNewMembersToUnread: true,
-    });
-  }
-  let ghostMemberPromise;
-  if (ghostMemberIDs) {
-    ghostMemberPromise = changeRole(id, ghostMemberIDs, -1);
-  }
+  const initialMemberPromise: Promise<?MembershipChangeset> = initialMemberIDs
+    ? changeRole(id, initialMemberIDs, null, { setNewMembersToUnread: true })
+    : Promise.resolve(undefined);
+  const ghostMemberPromise: Promise<?MembershipChangeset> = ghostMemberIDs
+    ? changeRole(id, ghostMemberIDs, -1)
+    : Promise.resolve(undefined);
 
   const [
     creatorChangeset,
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
@@ -68,7 +68,7 @@
   +oldRole: string,
 };
 type MembershipRow = MembershipRowToSave | MembershipRowToDelete;
-type Changeset = {
+export type MembershipChangeset = {
   +membershipRows: MembershipRow[],
   +relationshipChangeset: RelationshipChangeset,
 };
@@ -90,7 +90,7 @@
   userIDs: $ReadOnlyArray<string>,
   role: string | -1 | 0 | null,
   options?: ChangeRoleOptions,
-): Promise<Changeset> {
+): Promise<MembershipChangeset> {
   const intent = role === -1 || role === 0 ? 'leave' : 'join';
   const setNewMembersToUnread =
     options?.setNewMembersToUnread && intent === 'join';
@@ -415,7 +415,7 @@
 };
 async function updateDescendantPermissions(
   initialChangedAncestor: ChangedAncestor,
-): Promise<Changeset> {
+): Promise<MembershipChangeset> {
   const membershipRows = [];
   const relationshipChangeset = new RelationshipChangeset();
 
@@ -708,7 +708,7 @@
 };
 async function recalculateThreadPermissions(
   threadID: string,
-): Promise<Changeset> {
+): Promise<MembershipChangeset> {
   const threadQuery = SQL`
     SELECT type, depth, parent_thread_id, containing_thread_id
     FROM threads
@@ -1069,7 +1069,7 @@
 };
 async function commitMembershipChangeset(
   viewer: Viewer,
-  changeset: Changeset,
+  changeset: MembershipChangeset,
   {
     changedThreadIDs = new Set(),
     calendarQuery,