diff --git a/src/Message/Message.php b/src/Message/Message.php
index f26ad9d..753dcd1 100644
--- a/src/Message/Message.php
+++ b/src/Message/Message.php
@@ -158,19 +158,19 @@ abstract class Message implements MessageInterface
      * and supply your own delimiter when concatenating.
      *
      * If the header does not appear in the message, this method MUST return
-     * a null value.
+     * an empty string.
      *
      * @param string $name Case-insensitive header field name.
-     * @return string|null A string of values as provided for the given header
+     * @return string A string of values as provided for the given header
      *    concatenated together using a comma. If the header does not appear in
-     *    the message, this method MUST return a null value.
+     *    the message, this method MUST return an empty string.
      */
     public function getHeaderLine($name)
     {
         if (isset($this->headers[$name])) {
             return join(", ", $this->headers[$name]);
         } else {
-            return null;
+            return "";
         }
     }
 
diff --git a/test/tests/unit/Message/MessageTest.php b/test/tests/unit/Message/MessageTest.php
index 7e94eaa..6e575d7 100644
--- a/test/tests/unit/Message/MessageTest.php
+++ b/test/tests/unit/Message/MessageTest.php
@@ -186,10 +186,10 @@ class MessageTest extends \PHPUnit_Framework_TestCase
     /**
      * @covers WellRESTed\Message\Message::getHeaderLine
      */
-    public function testGetHeaderLineReturnsNullForUnsetHeader()
+    public function testGetHeaderLineReturnsEmptyStringForUnsetHeader()
     {
         $message = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
-        $this->assertNull($message->getHeaderLine("X-not-set"));
+        $this->assertSame("", $message->getHeaderLine("X-not-set"));
     }
 
     /**