diff --git a/lib/types/entry-types.js b/lib/types/entry-types.js
--- a/lib/types/entry-types.js
+++ b/lib/types/entry-types.js
@@ -19,7 +19,7 @@
   fifteenDaysLater,
   thisMonthDates,
 } from '../utils/date-utils.js';
-import { tID, tShape } from '../utils/validation-utils.js';
+import { tUserID, tID, tShape } from '../utils/validation-utils.js';
 
 export type RawEntryInfo = {
   id?: string, // null if local copy without ID yet
@@ -46,7 +46,7 @@
     month: t.Number,
     day: t.Number,
     creationTime: t.Number,
-    creatorID: t.String,
+    creatorID: tUserID,
     deleted: t.Boolean,
   });
 
diff --git a/lib/types/history-types.js b/lib/types/history-types.js
--- a/lib/types/history-types.js
+++ b/lib/types/history-types.js
@@ -2,7 +2,7 @@
 
 import t, { type TInterface } from 'tcomb';
 
-import { tID, tShape } from '../utils/validation-utils.js';
+import { tID, tShape, tUserID } from '../utils/validation-utils.js';
 
 export type HistoryMode = 'day' | 'entry';
 
@@ -19,7 +19,7 @@
   tShape<HistoryRevisionInfo>({
     id: tID,
     entryID: tID,
-    authorID: t.String,
+    authorID: tUserID,
     text: t.String,
     lastUpdate: t.Number,
     deleted: t.Boolean,
diff --git a/lib/types/user-types.js b/lib/types/user-types.js
--- a/lib/types/user-types.js
+++ b/lib/types/user-types.js
@@ -11,7 +11,7 @@
   type UserRelationshipStatus,
   userRelationshipStatusValidator,
 } from './relationship-types.js';
-import { tBool, tShape } from '../utils/validation-utils.js';
+import { tBool, tShape, tUserID } from '../utils/validation-utils.js';
 
 export type GlobalUserInfo = {
   +id: string,
@@ -57,7 +57,7 @@
 };
 export const accountUserInfoValidator: TInterface<AccountUserInfo> =
   tShape<AccountUserInfo>({
-    id: t.String,
+    id: tUserID,
     username: t.String,
     relationshipStatus: t.maybe(userRelationshipStatusValidator),
     avatar: t.maybe(clientAvatarValidator),
diff --git a/lib/types/validators/entry-validators.js b/lib/types/validators/entry-validators.js
--- a/lib/types/validators/entry-validators.js
+++ b/lib/types/validators/entry-validators.js
@@ -3,7 +3,7 @@
 import t from 'tcomb';
 import type { TInterface } from 'tcomb';
 
-import { tShape, tID } from '../../utils/validation-utils.js';
+import { tShape, tID, tUserID } from '../../utils/validation-utils.js';
 import {
   type DeleteEntryResponse,
   type RestoreEntryResponse,
@@ -23,7 +23,7 @@
 export const fetchEntryInfosResponseValidator: TInterface<FetchEntryInfosResponse> =
   tShape<FetchEntryInfosResponse>({
     rawEntryInfos: t.list(rawEntryInfoValidator),
-    userInfos: t.dict(t.String, accountUserInfoValidator),
+    userInfos: t.dict(tUserID, accountUserInfoValidator),
   });
 
 export const fetchEntryRevisionInfosResultValidator: TInterface<FetchEntryRevisionInfosResult> =