diff --git a/PasscodeAuth.ios.js b/PasscodeAuth.ios.js
index a1ea63e..0edb283 100644
--- a/PasscodeAuth.ios.js
+++ b/PasscodeAuth.ios.js
@@ -28,7 +28,7 @@ var PasscodeAuth = {
     });
   },
 
-  authenticate(reason) {
+  authenticate(reason: ?string) {
     var authReason;
 
     // Set auth reason