diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -927,7 +927,7 @@
 
 export const policyAcknowledgmentRequestInputValidator: TInterface<PolicyAcknowledgmentRequest> =
   tShape<PolicyAcknowledgmentRequest>({
-    policy: t.maybe(t.enums.of(policies)),
+    policy: t.enums.of(policies),
   });
 
 async function policyAcknowledgmentResponder(
diff --git a/lib/facts/policies.js b/lib/facts/policies.js
--- a/lib/facts/policies.js
+++ b/lib/facts/policies.js
@@ -7,10 +7,9 @@
 export const policyTypes = Object.freeze({
   tosAndPrivacyPolicy: 'TERMS_OF_USE_AND_PRIVACY_POLICY',
 });
-
-export const policies: $ReadOnlyArray<string> = values(policyTypes);
-
 export type PolicyType = $Values<typeof policyTypes>;
+export const policies: $ReadOnlyArray<PolicyType> = values(policyTypes);
+
 export const policyTypeValidator: TEnums = t.enums.of(policies);
 
 export const baseLegalPolicies = [policyTypes.tosAndPrivacyPolicy];
diff --git a/lib/types/validators/user-validators.js b/lib/types/validators/user-validators.js
--- a/lib/types/validators/user-validators.js
+++ b/lib/types/validators/user-validators.js
@@ -1,8 +1,8 @@
 // @flow
 
-import t, { type TInterface, type TUnion, type TEnums } from 'tcomb';
+import t, { type TInterface, type TUnion } from 'tcomb';
 
-import { policyTypeValidator } from '../../facts/policies.js';
+import { policyTypeValidator, type PolicyType } from '../../facts/policies.js';
 import { mixedThinRawThreadInfoValidator } from '../../permissions/minimally-encoded-raw-thread-info-validators.js';
 import { tShape, tID, tUserID } from '../../utils/validation-utils.js';
 import type {
@@ -60,7 +60,7 @@
       threadInfos: t.dict(tID, mixedThinRawThreadInfoValidator),
       userInfos: t.list(userInfoValidator),
     }),
-    notAcknowledgedPolicies: t.maybe(t.list<TEnums>(policyTypeValidator)),
+    notAcknowledgedPolicies: t.maybe(t.list<PolicyType>(policyTypeValidator)),
   });
 
 export const subscriptionUpdateResponseValidator: TInterface<SubscriptionUpdateResponse> =