diff --git a/src/Message/Message.php b/src/Message/Message.php
index 217bdc8..f39e8ac 100644
--- a/src/Message/Message.php
+++ b/src/Message/Message.php
@@ -20,6 +20,11 @@ abstract class Message implements MessageInterface
         $this->body = new NullStream();
     }
 
+    public function __clone()
+    {
+        $this->headers = clone $this->headers;
+    }
+
     // ------------------------------------------------------------------------
     // Psr\Http\Message\MessageInterface
 
@@ -171,9 +176,13 @@ abstract class Message implements MessageInterface
      */
     public function withHeader($name, $value)
     {
+        $values = $this->getValidatedHeaders($name, $value);
+
         $message = clone $this;
         unset($message->headers[$name]);
-        $message->headers[$name] = $value;
+        foreach ($values as $value) {
+            $message->headers[$name] = (string) $value;
+        }
         return $message;
     }
 
@@ -196,8 +205,12 @@ abstract class Message implements MessageInterface
      */
     public function withAddedHeader($name, $value)
     {
+        $values = $this->getValidatedHeaders($name, $value);
+
         $message = clone $this;
-        $message->headers[$name] = $value;
+        foreach ($values as $value) {
+            $message->headers[$name] = (string) $value;
+        }
         return $message;
     }
 
@@ -252,9 +265,23 @@ abstract class Message implements MessageInterface
 
     // ------------------------------------------------------------------------
 
-    public function __clone()
+    private function getValidatedHeaders($name, $value)
     {
-        $this->headers = clone $this->headers;
+        $is_allowed = function ($item) {
+            return is_string($item) || is_numeric($item);
+        };
+
+        if (!is_string($name)) {
+            throw new \InvalidArgumentException("Header name must be a string");
+        }
+
+        if ($is_allowed($value)) {
+            return [$value];
+        } elseif (is_array($value) && count($value) === count(array_filter($value, $is_allowed))) {
+            return $value;
+        } else {
+            throw new \InvalidArgumentException("Header values must be a string or string[]");
+        }
     }
 }
 
diff --git a/test/tests/unit/Message/MessageTest.php b/test/tests/unit/Message/MessageTest.php
index f4b1642..c294eaf 100644
--- a/test/tests/unit/Message/MessageTest.php
+++ b/test/tests/unit/Message/MessageTest.php
@@ -17,6 +17,21 @@ class MessageTest extends \PHPUnit_Framework_TestCase
         $this->assertNotNull($message);
     }
 
+    /**
+     * @covers WellRESTed\Message\Message::__clone
+     */
+    public function testCloneMakesDeepCopyOfHeaders()
+    {
+        $message1 = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
+        $message1 = $message1->withHeader("Content-type", "text/plain");
+        $message2 = $message1->withHeader("Content-type", "application/json");
+        $this->assertEquals(["text/plain"], $message1->getHeader("Content-type"));
+        $this->assertEquals(["application/json"], $message2->getHeader("Content-type"));
+    }
+
+    // ------------------------------------------------------------------------
+    // Protocol Version
+
     /**
      * @covers WellRESTed\Message\Message::getProtocolVersion
      */
@@ -46,27 +61,49 @@ class MessageTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals("1.0", $message->getProtocolVersion());
     }
 
+    // ------------------------------------------------------------------------
+    // Headers
+
     /**
      * @covers WellRESTed\Message\Message::withHeader
+     * @covers WellRESTed\Message\Message::getValidatedHeaders
+     * @dataProvider validHeaderValueProvider
      */
-    public function testWithHeaderSetsHeader()
+    public function testWithHeaderReplacesHeader($expected, $value)
     {
         $message = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
-        $message = $message->withHeader("Content-type", "application/json");
-        $this->assertEquals(["application/json"], $message->getHeader("Content-type"));
+        $message = $message->withHeader("X-foo", "Original value");
+        $message = $message->withHeader("X-foo", $value);
+        $this->assertEquals($expected, $message->getHeader("X-foo"));
+    }
+
+    public function validHeaderValueProvider()
+    {
+        return [
+            [["0"], 0],
+            [["molly","bear"],["molly","bear"]]
+        ];
     }
 
     /**
      * @covers WellRESTed\Message\Message::withHeader
+     * @covers WellRESTed\Message\Message::getValidatedHeaders
+     * @expectedException \InvalidArgumentException
+     * @dataProvider invalidHeaderProvider
      */
-    public function testWithHeaderReplacesValue()
+    public function testWithHeaderThrowExceptionWithInvalidArgument($name, $value)
     {
         $message = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
-        $message = $message->withHeader("Set-Cookie", "cat=Molly");
-        $message = $message->withHeader("Set-Cookie", "dog=Bear");
-        $cookies = $message->getHeader("Set-Cookie");
-        $this->assertNotContains("cat=Molly", $cookies);
-        $this->assertContains("dog=Bear", $cookies);
+        $message->withHeader($name, $value);
+    }
+
+    public function invalidHeaderProvider()
+    {
+        return [
+            [0, 1024],
+            ["Content-length", false],
+            ["Content-length", [false]],
+        ];
     }
 
     /**
@@ -85,8 +122,8 @@ class MessageTest extends \PHPUnit_Framework_TestCase
     public function testWithAddedHeaderAppendsValue()
     {
         $message = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
-        $message = $message->withAddedHeader("Set-Cookie", "cat=Molly");
-        $message = $message->withAddedHeader("Set-Cookie", "dog=Bear");
+        $message = $message->withAddedHeader("Set-Cookie", ["cat=Molly"]);
+        $message = $message->withAddedHeader("Set-Cookie", ["dog=Bear"]);
         $cookies = $message->getHeader("Set-Cookie");
         $this->assertContains("cat=Molly", $cookies);
         $this->assertContains("dog=Bear", $cookies);
@@ -222,6 +259,9 @@ class MessageTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expected, $headers);
     }
 
+    // ------------------------------------------------------------------------
+    // Body
+
     /**
      * @covers WellRESTed\Message\Message::getBody
      * @uses WellRESTed\Message\NullStream
@@ -245,16 +285,4 @@ class MessageTest extends \PHPUnit_Framework_TestCase
         $message = $message->withBody($stream);
         $this->assertSame($stream, $message->getBody());
     }
-
-    /**
-     * @covers WellRESTed\Message\Message::__clone
-     */
-    public function testCloneMakesDeepCopyOfHeaders()
-    {
-        $message1 = $this->getMockForAbstractClass('\WellRESTed\Message\Message');
-        $message1 = $message1->withHeader("Content-type", "text/plain");
-        $message2 = $message1->withHeader("Content-type", "application/json");
-        $this->assertEquals(["text/plain"], $message1->getHeader("Content-type"));
-        $this->assertEquals(["application/json"], $message2->getHeader("Content-type"));
-    }
 }