diff --git a/src/MessageTrait.php b/src/MessageTrait.php
index 831e20d7..b7cf98b5 100644
--- a/src/MessageTrait.php
+++ b/src/MessageTrait.php
@@ -138,6 +138,11 @@ private function setHeaders(array $headers)
     {
         $this->headerNames = $this->headers = [];
         foreach ($headers as $header => $value) {
+            if (is_int($header)) {
+                // Numeric array keys are converted to int by PHP but having a header name '123' is not forbidden by the spec
+                // and also allowed in withHeader(). So we need to cast it to string again for the following assertion to pass.
+                $header = (string) $header;
+            }
             $this->assertHeader($header);
             $value = $this->normalizeHeaderValue($value);
             $normalized = strtolower($header);
@@ -154,10 +159,6 @@ 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]);
         }
 
@@ -185,17 +186,28 @@ private function normalizeHeaderValue($value)
     private function trimHeaderValues(array $values)
     {
         return array_map(function ($value) {
-            if (!is_string($value)) {
-                throw new \InvalidArgumentException('Header value must be a string.');
+            if (!is_string($value) && !is_numeric($value)) {
+                throw new \InvalidArgumentException(sprintf(
+                    'Header value must be a string or numeric but %s provided.',
+                    is_object($value) ? get_class($value) : gettype($value)
+                ));
             }
-            return trim($value, " \t");
+
+            return trim((string) $value, " \t");
         }, $values);
     }
 
     private function assertHeader($header)
     {
-        if (!is_string($header) || $header === '') {
-            throw new \InvalidArgumentException('Header must be a non empty string.');
+        if (!is_string($header)) {
+            throw new \InvalidArgumentException(sprintf(
+                'Header name must be a string but %s provided.',
+                is_object($header) ? get_class($header) : gettype($header)
+            ));
+        }
+
+        if ($header === '') {
+            throw new \InvalidArgumentException('Header name can not be empty.');
         }
     }
 }
diff --git a/src/Request.php b/src/Request.php
index 355e546b..59f337db 100644
--- a/src/Request.php
+++ b/src/Request.php
@@ -145,7 +145,7 @@ private function updateHostFromUri()
     private function assertMethod($method)
     {
         if (!is_string($method) || $method === '') {
-            throw new \InvalidArgumentException('Method should be a non empty string.');
+            throw new \InvalidArgumentException('Method must be a non-empty string.');
         }
     }
 }
diff --git a/src/Response.php b/src/Response.php
index bea5f260..e7e04d86 100644
--- a/src/Response.php
+++ b/src/Response.php
@@ -97,7 +97,7 @@ public function __construct(
         $status = (int) $status;
         $this->assertStatusCodeRange($status);
 
-        $this->statusCode = (int) $status;
+        $this->statusCode = $status;
 
         if ($body !== '' && $body !== null) {
             $this->stream = stream_for($body);
diff --git a/tests/ResponseTest.php b/tests/ResponseTest.php
index 0fde5600..6496a8d0 100644
--- a/tests/ResponseTest.php
+++ b/tests/ResponseTest.php
@@ -237,6 +237,12 @@ public function testSameInstanceWhenRemovingMissingHeader()
         $this->assertSame($r, $r->withoutHeader('foo'));
     }
 
+    public function testPassNumericHeaderNameInConstructor()
+    {
+        $r = new Response(200, ['Location' => 'foo', '123' => 'bar']);
+        $this->assertSame('bar', $r->getHeaderLine('123'));
+    }
+
     /**
      * @dataProvider invalidHeaderProvider
      */
@@ -250,11 +256,9 @@ 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.'],
+            ['', '', 'Header name can not be empty.'],
+            ['foo', false, 'Header value must be a string or numeric but boolean provided'],
+            ['foo', new \stdClass(),  'Header value must be a string or numeric but stdClass provided.'],
         ];
     }
 
@@ -270,16 +274,11 @@ public function testWithInvalidHeader($header, $headerValue, $expectedMessage)
 
     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.'],
-        ];
+        return array_merge($this->invalidHeaderProvider(), [
+            [[], 'foo', 'Header name must be a string but array provided.'],
+            [false, 'foo', 'Header name must be a string but boolean provided.'],
+            [new \stdClass(), 'foo', 'Header name must be a string but stdClass provided.'],
+        ]);
     }
 
     public function testHeaderValuesAreTrimmed()