diff --git a/sha256.js b/sha256.js
index eb0b48a..28a1495 100644
--- a/sha256.js
+++ b/sha256.js
@@ -8,6 +8,6 @@
 const {NativeModules} = require('react-native')
 const sha256Lib = NativeModules.sha256Lib;
 
-export function sha256(data) {
+export function sha256(data: string) {
   return sha256Lib.sha256(data);
 }