diff --git a/lib/types/thread-permission-types.js b/lib/types/thread-permission-types.js
--- a/lib/types/thread-permission-types.js
+++ b/lib/types/thread-permission-types.js
@@ -1,10 +1,10 @@
 // @flow
 
 import invariant from 'invariant';
-import t, { type TDict } from 'tcomb';
+import t, { type TDict, type TUnion } from 'tcomb';
 
 import { values } from '../utils/objects.js';
-import { tBool, tShape } from '../utils/validation-utils.js';
+import { tBool, tShape, tID } from '../utils/validation-utils.js';
 
 export const threadPermissions = Object.freeze({
   KNOW_OF: 'know_of',
@@ -87,10 +87,11 @@
 export type ThreadPermissionInfo =
   | { +value: true, +source: string }
   | { +value: false, +source: null };
-const threadPermissionInfoValidator = t.union([
-  tShape({ value: tBool(true), source: t.String }),
-  tShape({ value: tBool(false), source: t.Nil }),
-]);
+export const threadPermissionInfoValidator: TUnion<ThreadPermissionInfo> =
+  t.union([
+    tShape({ value: tBool(true), source: tID }),
+    tShape({ value: tBool(false), source: t.Nil }),
+  ]);
 export type ThreadPermissionsBlob = {
   +[permission: string]: ThreadPermissionInfo,
 };