diff --git a/hmac.go b/hmac.go
index 2d7f1bf..402ff08 100644
--- a/hmac.go
+++ b/hmac.go
@@ -66,6 +66,8 @@ func (m *SigningMethodHMAC) Verify(signingString, signature string, key interfac
 	return ErrInvalidKey
 }
 
+// Implements the Sign method from SigningMethod for this signing method.
+// Key must be []byte
 func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) (string, error) {
 	if keyBytes, ok := key.([]byte); ok {
 		if !m.Hash.Available() {