diff --git a/Trustly/Api/api.php b/Trustly/Api/api.php
index c9fee7d..9bf914c 100644
--- a/Trustly/Api/api.php
+++ b/Trustly/Api/api.php
@@ -473,6 +473,9 @@ public function call($request) {
 		$this->last_request = $request;
 
 		$jsonstr = $request->json();
+		if($jsonstr === FALSE) {
+			throw new Trustly_DataException('Unable to json encode payload');
+		}
 
 		$url = $this->url($request);
 		list($body, $response_code) = $this->post($url, $jsonstr);
diff --git a/Trustly/Data/data.php b/Trustly/Data/data.php
index a77274b..e2f8643 100644
--- a/Trustly/Data/data.php
+++ b/Trustly/Data/data.php
@@ -158,7 +158,7 @@ public function pop($name) {
 	 * @param boolean $pretty Format the output in a prettified easy-to-read
 	 *		formatting
 	 *
-	 * @return string The current payload in JSON
+	 * @return string|false The current payload in JSON
 	 */
 	public function json($pretty=FALSE) {
 		if($pretty) {