diff --git a/lib/types/identity-service-types.js b/lib/types/identity-service-types.js
--- a/lib/types/identity-service-types.js
+++ b/lib/types/identity-service-types.js
@@ -374,7 +374,9 @@
 
 export type IdentityDeviceType = $Values<typeof identityDeviceTypes>;
 
-function isIdentityDeviceType(deviceType: number): boolean {
+function isIdentityDeviceType(
+  deviceType: number,
+): deviceType is IdentityDeviceType {
   return (
     deviceType === 0 ||
     deviceType === 1 ||