diff --git a/lib/types/validators/relationship-validators.js b/lib/types/validators/relationship-validators.js
--- a/lib/types/validators/relationship-validators.js
+++ b/lib/types/validators/relationship-validators.js
@@ -2,12 +2,12 @@
 
 import t, { type TInterface } from 'tcomb';
 
-import { tShape } from '../../utils/validation-utils.js';
+import { tShape, tUserID } from '../../utils/validation-utils.js';
 import { type RelationshipErrors } from '../relationship-types.js';
 
 export const relationshipErrorsValidator: TInterface<RelationshipErrors> =
   tShape<RelationshipErrors>({
-    invalid_user: t.maybe(t.list(t.String)),
-    already_friends: t.maybe(t.list(t.String)),
-    user_blocked: t.maybe(t.list(t.String)),
+    invalid_user: t.maybe(t.list(tUserID)),
+    already_friends: t.maybe(t.list(tUserID)),
+    user_blocked: t.maybe(t.list(tUserID)),
   });