diff --git a/src/Face/Traits/Faceb2bTrait.php b/src/Face/Traits/Faceb2bTrait.php
index c5937d6..8a90a7a 100644
--- a/src/Face/Traits/Faceb2bTrait.php
+++ b/src/Face/Traits/Faceb2bTrait.php
@@ -80,7 +80,7 @@ public function requestInvoiceCancellation($regId, $reason, $comment=null) {
    */
   public function getRegisteredInvoices($receivingUnit=null) {
     $req = '<web:GetRegisteredInvoices><request>';
-    if (is_null($receivingUnit)) {
+    if (!is_null($receivingUnit)) {
       $req .= '<receivingUnit>' . $receivingUnit . '</receivingUnit>';
     }
     $req .= '</request></web:GetRegisteredInvoices>';