diff --git a/src/System.php b/src/System.php
index 4f70247..9f6a4d0 100644
--- a/src/System.php
+++ b/src/System.php
@@ -1559,6 +1559,9 @@ private function addOAuth1Signature($endpoint, $data, $method, $type, $hash, $ti
 
         if (!is_array($data)) {
             if (empty($hash)) {  // Calculate body hash
+                if (is_null($data)) {
+                    $data = '';
+                }
                 switch ($this->signatureMethod) {
                     case 'HMAC-SHA224':
                         $hash = base64_encode(hash('sha224', $data, true));