diff --git a/src/Message/SharedCaptureRequest.php b/src/Message/SharedCaptureRequest.php
index a655e94..d1cd18e 100644
--- a/src/Message/SharedCaptureRequest.php
+++ b/src/Message/SharedCaptureRequest.php
@@ -44,7 +44,11 @@ public function getData()
 
             // The documentation (2015) says this is required. But it can't be, because
             // we won't have it for authenticate, as the bank has not been visited.
-            //$data['RelatedTxAuthNo'] = $this->getTxAuthNo();
+            // We will follow the spec though, but treat it as optional here.
+
+            if ($this->getTxAuthNo() !== null) {
+                $data['RelatedTxAuthNo'] = $this->getTxAuthNo();
+            }
         } else {
             $this->validate('txAuthNo');
 
diff --git a/src/Message/SharedRepeatAuthorizeRequest.php b/src/Message/SharedRepeatAuthorizeRequest.php
index e358d31..68e9797 100644
--- a/src/Message/SharedRepeatAuthorizeRequest.php
+++ b/src/Message/SharedRepeatAuthorizeRequest.php
@@ -57,7 +57,7 @@ public function getData()
 
         $data['Description'] = $this->getDescription();
 
-        // SagePay's unique reference for the PREVIOUS transaction
+        // Sage Pay's unique reference for the ORIGINAL transaction
 
         $data['RelatedVendorTxCode'] = $this->getRelatedTransactionId();
         $data['RelatedVPSTxId'] = $this->getVpsTxId();
diff --git a/tests/Message/SharedCaptureRequestTest.php b/tests/Message/SharedCaptureRequestTest.php
new file mode 100644
index 0000000..0035709
--- /dev/null
+++ b/tests/Message/SharedCaptureRequestTest.php
@@ -0,0 +1,104 @@
+<?php
+
+namespace Omnipay\SagePay\Message;
+
+use Omnipay\Tests\TestCase;
+
+class SharedCaptureRequestTest extends TestCase
+{
+    public function setUp()
+    {
+        parent::setUp();
+
+        $this->request = new SharedCaptureRequest($this->getHttpClient(), $this->getHttpRequest());
+        $this->request->initialize(
+            array(
+                'transactionReference' => '{"SecurityKey":"JEUPDN1N7E","TxAuthNo":"4255","VPSTxId":"{F955C22E-F67B-4DA3-8EA3-6DAC68FA59D2}","VendorTxCode":"438791"}',
+                'testMode' => true,
+            )
+        );
+    }
+
+    public function testTxType()
+    {
+        // Default TxType is RELEASE.
+
+        $this->assertSame('RELEASE', $this->request->getTxType());
+
+        // User authenticate explicity true.
+
+        $this->request->setUseAuthenticate(true);
+
+        $this->assertSame('AUTHORISE', $this->request->getTxType());
+
+        // User authenticate explicity false (back to the default).
+
+        $this->request->setUseAuthenticate(false);
+
+        $this->assertSame('RELEASE', $this->request->getTxType());
+    }
+
+    /**
+     * @expectedException \Omnipay\Common\Exception\InvalidRequestException
+     */
+    public function testMissingAmount()
+    {
+        $this->request->getData();
+    }
+
+    public function testValid()
+    {
+        $this->request->setAmount(123.45);
+
+        $data = $this->request->getData();
+
+        $this->assertSame('123.45', $data['ReleaseAmount']);
+        $this->assertSame('438791', $data['VendorTxCode']);
+
+        $this->assertSame('{F955C22E-F67B-4DA3-8EA3-6DAC68FA59D2}', $data['VPSTxId']);
+        $this->assertSame('JEUPDN1N7E', $data['SecurityKey']);
+        $this->assertSame('4255', $data['TxAuthNo']);
+    }
+
+    /**
+     * @expectedException \Omnipay\Common\Exception\InvalidRequestException
+     */
+    public function testAuthMissingDescription()
+    {
+        $this->request->setAmount(123.45);
+        $this->request->setUseAuthenticate(true);
+
+        $this->request->getData();
+    }
+
+    /**
+     * @expectedException \Omnipay\Common\Exception\InvalidRequestException
+     */
+    public function testAuthMissingTransactionId()
+    {
+        $this->request->setAmount(123.45);
+        $this->request->setDescription('desc');
+        $this->request->setUseAuthenticate(true);
+
+        $this->request->getData();
+    }
+
+    public function testAuthValid()
+    {
+        $this->request->setAmount(123.45);
+        $this->request->setDescription('desc');
+        $this->request->setTransactionId('438791-NEW');
+        $this->request->setUseAuthenticate(true);
+
+        $data = $this->request->getData();
+
+        $this->assertSame('123.45', $data['Amount']);
+        $this->assertSame('438791-NEW', $data['VendorTxCode']);
+        $this->assertSame('desc', $data['Description']);
+
+        $this->assertSame('438791', $data['RelatedVendorTxCode']);
+        $this->assertSame('{F955C22E-F67B-4DA3-8EA3-6DAC68FA59D2}', $data['RelatedVPSTxId']);
+        $this->assertSame('JEUPDN1N7E', $data['RelatedSecurityKey']);
+        $this->assertSame('4255', $data['RelatedTxAuthNo']);
+    }
+}