diff --git a/src/Attachment.php b/src/Attachment.php
index 50103a3..d3c717e 100644
--- a/src/Attachment.php
+++ b/src/Attachment.php
@@ -161,25 +161,32 @@ public function xmlSerialize(Writer $writer): void
             $fileContents = base64_encode(file_get_contents($this->filePath));
             $fileName = basename($this->filePath);
             $mimeType = $this->getFilePathMimeType();
-        } else {
+
+            $writer->write([
+               'name'       => Schema::CBC . 'EmbeddedDocumentBinaryObject',
+               'value'      => $fileContents,
+               'attributes' => [
+                   'mimeCode' => $mimeType,
+                   'filename' => $fileName,
+               ],
+           ]);
+        } elseif (!empty($this->base64Content)) {
             $fileContents = $this->base64Content;
             $fileName = $this->fileName;
             $mimeType = $this->mimeType;
-        }
-
-        $writer->write([
-            'name'       => Schema::CBC . 'EmbeddedDocumentBinaryObject',
-            'value'      => $fileContents,
-            'attributes' => [
-                'mimeCode' => $mimeType,
-                'filename' => $fileName,
-            ]
-        ]);
 
-        if ($this->externalReference) {
+            $writer->write([
+               'name'       => Schema::CBC . 'EmbeddedDocumentBinaryObject',
+               'value'      => $fileContents,
+               'attributes' => [
+                   'mimeCode' => $mimeType,
+                   'filename' => $fileName,
+               ],
+           ]);
+        } elseif (!empty($this->externalReference)) {
             $writer->writeElement(
                 Schema::CAC . 'ExternalReference',
-                [ Schema::CBC . 'URI' => $this->externalReference ]
+                [Schema::CBC . 'URI' => $this->externalReference]
             );
         }
     }
diff --git a/tests/Write/SimpleInvoiceWithExternalReferenceTest.php b/tests/Write/SimpleInvoiceWithExternalReferenceTest.php
new file mode 100644
index 0000000..093b4cd
--- /dev/null
+++ b/tests/Write/SimpleInvoiceWithExternalReferenceTest.php
@@ -0,0 +1,170 @@
+<?php
+
+namespace NumNum\UBL\Tests\Write;
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Test an UBL2.1 invoice document
+ */
+class SimpleInvoiceWithExternalReferenceTest extends TestCase
+{
+    private $schema = 'http://docs.oasis-open.org/ubl/os-UBL-2.1/xsd/maindoc/UBL-Invoice-2.1.xsd';
+
+    /** @test */
+    public function testIfXMLIsValid()
+    {
+        // Address country
+        $country = (new \NumNum\UBL\Country())
+            ->setIdentificationCode('BE');
+
+        // Full address
+        $address = (new \NumNum\UBL\Address())
+            ->setStreetName('Korenmarkt')
+            ->setBuildingNumber(1)
+            ->setCityName('Gent')
+            ->setPostalZone('9000')
+            ->setCountry($country);
+
+        // Supplier company node
+        $supplierCompany = (new \NumNum\UBL\Party())
+            ->setName('Supplier Company Name')
+            ->setPhysicalLocation($address)
+            ->setPostalAddress($address);
+
+        // Client contact node
+        $clientContact = (new \NumNum\UBL\Contact())
+            ->setName('Client name')
+            ->setElectronicMail('email@client.com')
+            ->setTelephone('0032 472 123 456')
+            ->setTelefax('0032 9 1234 567');
+
+        // Client company node
+        $clientCompany = (new \NumNum\UBL\Party())
+            ->setName('My client')
+            ->setPostalAddress($address)
+            ->setContact($clientContact);
+
+        $legalMonetaryTotal = (new \NumNum\UBL\LegalMonetaryTotal())
+            ->setPayableAmount(10 + 2)
+            ->setAllowanceTotalAmount(0);
+
+        // Tax scheme
+        $taxScheme = (new \NumNum\UBL\TaxScheme())
+            ->setId(0);
+
+        // Product
+        $productItem = (new \NumNum\UBL\Item())
+            ->setName('Product Name')
+            ->setDescription('Product Description')
+            ->setSellersItemIdentification('SELLERID');
+
+        // Price
+        $price = (new \NumNum\UBL\Price())
+            ->setBaseQuantity(1)
+            ->setUnitCode(\NumNum\UBL\UnitCode::UNIT)
+            ->setPriceAmount(10);
+
+        // Invoice Line tax totals
+        $lineTaxTotal = (new \NumNum\UBL\TaxTotal())
+            ->setTaxAmount(2.1);
+
+        // InvoicePeriod
+        $invoicePeriod = (new \NumNum\UBL\InvoicePeriod())
+            ->setStartDate(new \DateTime());
+
+        // Invoice Line(s)
+        $invoiceLines = [];
+
+        $orderLineReference = (new \NumNum\UBL\OrderLineReference())
+            ->setLineId('#ABC123');
+
+        $invoiceLines[] = (new \NumNum\UBL\InvoiceLine())
+            ->setId(0)
+            ->setItem($productItem)
+            ->setInvoicePeriod($invoicePeriod)
+            ->setPrice($price)
+            ->setTaxTotal($lineTaxTotal)
+            ->setInvoicedQuantity(1)
+            ->setOrderLineReference($orderLineReference);
+
+        $invoiceLines[] = (new \NumNum\UBL\InvoiceLine())
+            ->setId(0)
+            ->setItem($productItem)
+            ->setInvoicePeriod($invoicePeriod)
+            ->setPrice($price)
+            ->setAccountingCost('Product 123')
+            ->setTaxTotal($lineTaxTotal)
+            ->setInvoicedQuantity(1)
+            ->setOrderLineReference($orderLineReference);
+
+        $invoiceLines[] = (new \NumNum\UBL\InvoiceLine())
+            ->setId(0)
+            ->setItem($productItem)
+            ->setInvoicePeriod($invoicePeriod)
+            ->setPrice($price)
+            ->setAccountingCostCode('Product 123')
+            ->setTaxTotal($lineTaxTotal)
+            ->setInvoicedQuantity(1)
+            ->setOrderLineReference($orderLineReference);
+
+
+        // Total Taxes
+        $taxCategory = (new \NumNum\UBL\TaxCategory())
+            ->setId(0)
+            ->setName('VAT21%')
+            ->setPercent(.21)
+            ->setTaxScheme($taxScheme);
+
+        $taxSubTotal = (new \NumNum\UBL\TaxSubTotal())
+            ->setTaxableAmount(10)
+            ->setTaxAmount(2.1)
+            ->setTaxCategory($taxCategory);
+
+
+        $taxTotal = (new \NumNum\UBL\TaxTotal())
+            ->addTaxSubTotal($taxSubTotal)
+            ->setTaxAmount(2.1);
+
+        $attachment = (new \NumNum\UBL\Attachment())
+            ->setExternalReference('http://www.suppliersite.eu/sheet001.html');
+
+        $additionalDocumentReference = (new \NumNum\UBL\AdditionalDocumentReference())
+            ->setId('Doc1')
+            ->setDocumentType('Timesheet')
+            ->setAttachment($attachment);
+
+        $accountingSupplierParty = (new \NumNum\UBL\AccountingParty())
+            ->setParty($supplierCompany);
+
+        $accountingCustomerParty = (new \NumNum\UBL\AccountingParty())
+            ->setSupplierAssignedAccountId('10001')
+            ->setParty($clientCompany);
+
+        // Invoice object
+        $invoice = (new \NumNum\UBL\Invoice())
+            ->setId(1234)
+            ->setAdditionalDocumentReference($additionalDocumentReference)
+            ->setCopyIndicator(false)
+            ->setIssueDate(new \DateTime())
+            ->setAccountingSupplierParty($accountingSupplierParty)
+            ->setAccountingCustomerParty($accountingCustomerParty)
+            ->setInvoiceLines($invoiceLines)
+            ->setLegalMonetaryTotal($legalMonetaryTotal)
+            ->setTaxTotal($taxTotal);
+
+        // Test created object
+        // Use \NumNum\UBL\Generator to generate an XML string
+        $generator = new \NumNum\UBL\Generator();
+        $outputXMLString = $generator->invoice($invoice);
+
+        // Create PHP Native DomDocument object, that can be
+        // used to validate the generate XML
+        $dom = new \DOMDocument();
+        $dom->loadXML($outputXMLString);
+
+        $dom->save('./tests/SimpleInvoiceWithExternalReferenceTest.xml');
+
+        $this->assertEquals(true, $dom->schemaValidate($this->schema));
+    }
+}