diff --git a/src/MessageTrait.php b/src/MessageTrait.php
index 1e4da649..831e20d7 100644
--- a/src/MessageTrait.php
+++ b/src/MessageTrait.php
@@ -66,11 +66,8 @@ public function getHeaderLine($header)
 
     public function withHeader($header, $value)
     {
-        if (!is_array($value)) {
-            $value = [$value];
-        }
-
-        $value = $this->trimHeaderValues($value);
+        $this->assertHeader($header);
+        $value = $this->normalizeHeaderValue($value);
         $normalized = strtolower($header);
 
         $new = clone $this;
@@ -85,11 +82,8 @@ public function withHeader($header, $value)
 
     public function withAddedHeader($header, $value)
     {
-        if (!is_array($value)) {
-            $value = [$value];
-        }
-
-        $value = $this->trimHeaderValues($value);
+        $this->assertHeader($header);
+        $value = $this->normalizeHeaderValue($value);
         $normalized = strtolower($header);
 
         $new = clone $this;
@@ -144,11 +138,8 @@ private function setHeaders(array $headers)
     {
         $this->headerNames = $this->headers = [];
         foreach ($headers as $header => $value) {
-            if (!is_array($value)) {
-                $value = [$value];
-            }
-
-            $value = $this->trimHeaderValues($value);
+            $this->assertHeader($header);
+            $value = $this->normalizeHeaderValue($value);
             $normalized = strtolower($header);
             if (isset($this->headerNames[$normalized])) {
                 $header = $this->headerNames[$normalized];
@@ -160,6 +151,23 @@ private function setHeaders(array $headers)
         }
     }
 
+    private function normalizeHeaderValue($value)
+    {
+        if (!is_array($value)) {
+            if (!is_string($value)) {
+                throw new \InvalidArgumentException('Header value must be a string or an array of strings.');
+            }
+
+            return $this->trimHeaderValues([$value]);
+        }
+
+        if (count($value) === 0) {
+            throw new \InvalidArgumentException('Header value can not be an empty array.');
+        }
+
+        return $this->trimHeaderValues($value);
+    }
+
     /**
      * Trims whitespace from the header values.
      *
@@ -177,7 +185,17 @@ private function setHeaders(array $headers)
     private function trimHeaderValues(array $values)
     {
         return array_map(function ($value) {
+            if (!is_string($value)) {
+                throw new \InvalidArgumentException('Header value must be a string.');
+            }
             return trim($value, " \t");
         }, $values);
     }
+
+    private function assertHeader($header)
+    {
+        if (!is_string($header) || $header === '') {
+            throw new \InvalidArgumentException('Header must be a non empty string.');
+        }
+    }
 }
diff --git a/src/Request.php b/src/Request.php
index 00066424..355e546b 100644
--- a/src/Request.php
+++ b/src/Request.php
@@ -36,6 +36,7 @@ public function __construct(
         $body = null,
         $version = '1.1'
     ) {
+        $this->assertMethod($method);
         if (!($uri instanceof UriInterface)) {
             $uri = new Uri($uri);
         }
@@ -91,6 +92,7 @@ public function getMethod()
 
     public function withMethod($method)
     {
+        $this->assertMethod($method);
         $new = clone $this;
         $new->method = strtoupper($method);
         return $new;
@@ -139,4 +141,11 @@ private function updateHostFromUri()
         // See: http://tools.ietf.org/html/rfc7230#section-5.4
         $this->headers = [$header => [$host]] + $this->headers;
     }
+
+    private function assertMethod($method)
+    {
+        if (!is_string($method) || $method === '') {
+            throw new \InvalidArgumentException('Method should be a non empty string.');
+        }
+    }
 }
diff --git a/src/Response.php b/src/Response.php
index 6e72c06b..bea5f260 100644
--- a/src/Response.php
+++ b/src/Response.php
@@ -93,9 +93,9 @@ public function __construct(
         $version = '1.1',
         $reason = null
     ) {
-        if (filter_var($status, FILTER_VALIDATE_INT) === false) {
-            throw new \InvalidArgumentException('Status code must be an integer value.');
-        }
+        $this->assertStatusCodeIsInteger($status);
+        $status = (int) $status;
+        $this->assertStatusCodeRange($status);
 
         $this->statusCode = (int) $status;
 
@@ -125,12 +125,30 @@ public function getReasonPhrase()
 
     public function withStatus($code, $reasonPhrase = '')
     {
+        $this->assertStatusCodeIsInteger($code);
+        $code = (int) $code;
+        $this->assertStatusCodeRange($code);
+
         $new = clone $this;
-        $new->statusCode = (int) $code;
+        $new->statusCode = $code;
         if ($reasonPhrase == '' && isset(self::$phrases[$new->statusCode])) {
             $reasonPhrase = self::$phrases[$new->statusCode];
         }
         $new->reasonPhrase = $reasonPhrase;
         return $new;
     }
+
+    private function assertStatusCodeIsInteger($statusCode)
+    {
+        if (filter_var($statusCode, FILTER_VALIDATE_INT) === false) {
+            throw new \InvalidArgumentException('Status code must be an integer value.');
+        }
+    }
+
+    private function assertStatusCodeRange($statusCode)
+    {
+        if ($statusCode < 100 || $statusCode >= 600) {
+            throw new \InvalidArgumentException('Status code must be an integer value between 1xx and 5xx.');
+        }
+    }
 }
diff --git a/tests/RequestTest.php b/tests/RequestTest.php
index 2e3969ca..c9b5957b 100644
--- a/tests/RequestTest.php
+++ b/tests/RequestTest.php
@@ -6,6 +6,7 @@
 use GuzzleHttp\Psr7\Uri;
 
 /**
+ * @covers GuzzleHttp\Psr7\MessageTrait
  * @covers GuzzleHttp\Psr7\Request
  */
 class RequestTest extends BaseTest
@@ -90,6 +91,35 @@ public function testWithUri()
         $this->assertSame($u1, $r1->getUri());
     }
 
+    /**
+     * @dataProvider invalidMethodsProvider
+     */
+    public function testConstructWithInvalidMethods($method)
+    {
+        $this->expectException('InvalidArgumentException');
+        new Request($method, '/');
+    }
+
+    /**
+     * @dataProvider invalidMethodsProvider
+     */
+    public function testWithInvalidMethods($method)
+    {
+        $r = new Request('get', '/');
+        $this->expectException('InvalidArgumentException');
+        $r->withMethod($method);
+    }
+
+    public function invalidMethodsProvider()
+    {
+        return [
+            [null],
+            [false],
+            [['foo']],
+            [new \stdClass()],
+        ];
+    }
+
     public function testSameInstanceWhenSameUri()
     {
         $r1 = new Request('GET', 'http://foo.com');
diff --git a/tests/ResponseTest.php b/tests/ResponseTest.php
index 4e9a37e1..0fde5600 100644
--- a/tests/ResponseTest.php
+++ b/tests/ResponseTest.php
@@ -237,6 +237,51 @@ public function testSameInstanceWhenRemovingMissingHeader()
         $this->assertSame($r, $r->withoutHeader('foo'));
     }
 
+    /**
+     * @dataProvider invalidHeaderProvider
+     */
+    public function testConstructResponseInvalidHeader($header, $headerValue, $expectedMessage)
+    {
+        $this->expectException('InvalidArgumentException', $expectedMessage);
+        new Response(200, [$header => $headerValue]);
+    }
+
+    public function invalidHeaderProvider()
+    {
+        return [
+            ['foo', [], 'Header value can not be an empty array.'],
+            ['', '', 'Header must be a non empty string.'],
+            ['foo', false, 'Header value must be a string or an array of strings.'],
+            [false, 'foo', 'Header must be a non empty string.'],
+            ['foo', new \stdClass(), 'Header value must be a string or an array of strings.'],
+            ['foo', new \ArrayObject(), 'Header value must be a string or an array of strings.'],
+        ];
+    }
+
+    /**
+     * @dataProvider invalidWithHeaderProvider
+     */
+    public function testWithInvalidHeader($header, $headerValue, $expectedMessage)
+    {
+        $r = new Response();
+        $this->expectException('InvalidArgumentException', $expectedMessage);
+        $r->withHeader($header, $headerValue);
+    }
+
+    public function invalidWithHeaderProvider()
+    {
+        return [
+            [[], 'foo', 'Header must be a non empty string.'],
+            ['foo', [], 'Header value can not be an empty array.'],
+            ['', '', 'Header must be a non empty string.'],
+            ['foo', false, 'Header value must be a string or an array of strings.'],
+            [false, 'foo', 'Header must be a non empty string.'],
+            ['foo', new \stdClass(), 'Header value must be a string or an array of strings.'],
+            ['foo', new \ArrayObject(), 'Header value must be a string or an array of strings.'],
+            [new \stdClass(), 'foo', 'Header must be a non empty string.'],
+        ];
+    }
+
     public function testHeaderValuesAreTrimmed()
     {
         $r1 = new Response(200, ['OWS' => " \t \tFoo\t \t "]);
@@ -251,16 +296,27 @@ public function testHeaderValuesAreTrimmed()
     }
 
     /**
-     * @dataProvider responseInitializedWithNonIntegerStatusCodeProvider
+     * @dataProvider nonIntegerStatusCodeProvider
      * @param mixed $invalidValues
      */
-    public function testResponseInitializedWithNonIntegerStatusCodeProvider($invalidValues)
+    public function testConstructResponseWithNonIntegerStatusCode($invalidValues)
     {
         $this->expectException('InvalidArgumentException', 'Status code must be an integer value.');
         new Response($invalidValues);
     }
 
-    public function responseInitializedWithNonIntegerStatusCodeProvider()
+    /**
+     * @dataProvider nonIntegerStatusCodeProvider
+     * @param mixed $invalidValues
+     */
+    public function testResponseChangeStatusCodeWithNonInteger($invalidValues)
+    {
+        $response = new Response();
+        $this->expectException('InvalidArgumentException', 'Status code must be an integer value.');
+        $response->withStatus($invalidValues);
+    }
+
+    public function nonIntegerStatusCodeProvider()
     {
         return [
             ['whatever'],
@@ -269,4 +325,33 @@ public function responseInitializedWithNonIntegerStatusCodeProvider()
             [new \stdClass()],
         ];
     }
+
+    /**
+     * @dataProvider invalidStatusCodeRangeProvider
+     * @param mixed $invalidValues
+     */
+    public function testConstructResponseWithInvalidRangeStatusCode($invalidValues)
+    {
+        $this->expectException('InvalidArgumentException', 'Status code must be an integer value between 1xx and 5xx.');
+        new Response($invalidValues);
+    }
+
+    /**
+     * @dataProvider invalidStatusCodeRangeProvider
+     * @param mixed $invalidValues
+     */
+    public function testResponseChangeStatusCodeWithWithInvalidRange($invalidValues)
+    {
+        $response = new Response();
+        $this->expectException('InvalidArgumentException', 'Status code must be an integer value between 1xx and 5xx.');
+        $response->withStatus($invalidValues);
+    }
+
+    public function invalidStatusCodeRangeProvider()
+    {
+        return [
+            [600],
+            [99],
+        ];
+    }
 }