diff --git a/Trustly/Api/signed.php b/Trustly/Api/signed.php
index 69d0b5b..40af480 100644
--- a/Trustly/Api/signed.php
+++ b/Trustly/Api/signed.php
@@ -107,7 +107,9 @@ public function loadMerchantPrivateKey($filename) {
 	 *
 	 * @see https://eu.developers.trustly.com/doc/reference/authentication
 	 *
-	 * @param string $cert Loaded private RSA key as a string
+	 * @param string|false $cert Loaded private RSA key as a string
+	 *
+	 * @return bool
 	 */
 	public function useMerchantPrivateKey($cert) {
 		if($cert !== FALSE) {