diff --git a/sha1-hmac.js b/sha1-hmac.js
index 3ae4163..fb107ee 100644
--- a/sha1-hmac.js
+++ b/sha1-hmac.js
@@ -56,7 +56,7 @@ exports.sha1Hmac = function (key, bytes) {
     var forge = window.forge;
     var hmac = forge.hmac.create();
     var digest;
-    hmac.start('sha1', key);
+    hmac.start('sha1', Unibabel.bufferToBinaryString(key));
     hmac.update(Unibabel.bufferToBinaryString(bytes));
     digest = hmac.digest().toHex();