diff --git a/src/Thalent/AcumulusPhp/InvoiceBuilder.php b/src/Thalent/AcumulusPhp/InvoiceBuilder.php
index d4c59bf..cca3527 100644
--- a/src/Thalent/AcumulusPhp/InvoiceBuilder.php
+++ b/src/Thalent/AcumulusPhp/InvoiceBuilder.php
@@ -84,6 +84,7 @@ public function build() {
 		$customer = new \SimpleXMLElement('<customer></customer>');
 
 		$customer->addChild('type', $this->customer->getContacttype());
+		$customer->addChild('contactid', $this->customer->getContactid());
 		$customer->addChild('overwriteifexists', $this->customer->getOverwriteifexists());
 		$customer->addChild('companyname1', $this->customer->getContactname1());
 		$customer->addChild('companyname2', $this->customer->getContactname2());