diff --git a/src/cloud.cls.php b/src/cloud.cls.php
index 43b3241f9..e598ae4f5 100644
--- a/src/cloud.cls.php
+++ b/src/cloud.cls.php
@@ -444,7 +444,7 @@ private function _sign_b64($data)
 
 			return false;
 		}
-		$signature = sodium_crypto_sign_detached($data, $sk);
+		$signature = sodium_crypto_sign_detached((string)$data, $sk);
 		return base64_encode($signature);
 	}