diff --git a/src/Qbil/Models/Invoice.php b/src/Qbil/Models/Invoice.php
index 172f8d9..5185b7f 100644
--- a/src/Qbil/Models/Invoice.php
+++ b/src/Qbil/Models/Invoice.php
@@ -15,7 +15,7 @@ class Invoice implements InvoiceInterface
     public function __construct(Document $document, bool $includeInvoiceLines = true)
     {
         $this->relation = Util::extract($document->Parties, 'supplier', 'ExternalId');
-        $this->invoiceType = Util::extract($document->HeaderFields, 'creditinvoice');
+        $this->creditInvoice = Util::extract($document->HeaderFields, 'creditinvoice');
         $this->subsidiary = Util::extract($document->Parties, 'buyer', 'ExternalId');
         $this->supplierInvoiceNumber = Util::extract($document->HeaderFields, 'invoicenumber');
         $this->amount = Util::extract($document->HeaderFields, 'invoicetotalvatexcludedamount');
@@ -51,7 +51,7 @@ public function __construct(Document $document, bool $includeInvoiceLines = true
     }
 
     private $relation;
-    private $invoiceType;
+    private $creditInvoice;
     private $subsidiary;
     private $supplierInvoiceNumber;
     private $amount;
@@ -187,9 +187,9 @@ public function getDieselSurcharge()
     /**
      * @return mixed
      */
-    public function getInvoiceType()
+    public function isCreditInvoice()
     {
-        return $this->invoiceType;
+        return 'true' === $this->creditInvoice;
     }
 
     public function getAmount()
diff --git a/src/Qbil/Models/InvoiceInterface.php b/src/Qbil/Models/InvoiceInterface.php
index 5b643e7..2257388 100644
--- a/src/Qbil/Models/InvoiceInterface.php
+++ b/src/Qbil/Models/InvoiceInterface.php
@@ -8,7 +8,7 @@ interface InvoiceInterface
 {
     public function addInvoiceLine(InvoiceLineInterface $invoiceLine);
     public function getRelation();
-    public function getInvoiceType();
+    public function isCreditInvoice();
     public function getSubsidiary();
     public function getSupplierInvoiceNumber();
     public function getAmount();