diff --git a/flight/net/Response.php b/flight/net/Response.php
index d5d3afa..1798de5 100644
--- a/flight/net/Response.php
+++ b/flight/net/Response.php
@@ -341,6 +341,15 @@ class Response
             );
         }
 
+        if ($this->content_length === true) {
+            // Send content length
+            $length = $this->getContentLength();
+
+            if ($length > 0) {
+                $this->setHeader('Content-Length', (string) $length);
+            }
+        }
+
         // Send other headers
         foreach ($this->headers as $field => $value) {
             if (\is_array($value)) {
@@ -352,15 +361,6 @@ class Response
             }
         }
 
-        if ($this->content_length) {
-            // Send content length
-            $length = $this->getContentLength();
-
-            if ($length > 0) {
-                $this->setRealHeader('Content-Length: ' . $length);
-            }
-        }
-
         return $this;
     }
 
@@ -437,7 +437,7 @@ class Response
             $this->processResponseCallbacks();
         }
 
-        if (!headers_sent()) {
+        if (headers_sent() === false) {
             $this->sendHeaders(); // @codeCoverageIgnore
         }
 
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index bafe06d..79f3ff0 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -26,6 +26,7 @@
         <exclude name="Generic.Commenting.DocComment.ContentBeforeClose" />
         <exclude name="Generic.Commenting.DocComment.MissingShort" />
         <exclude name="Generic.Commenting.DocComment.SpacingBeforeShort" />
+        <exclude name="Generic.Formatting.NoSpaceAfterCast.SpaceFound" />
         <exclude name="Generic.Functions.OpeningFunctionBraceKernighanRitchie.BraceOnNewLine" />
         <exclude name="Generic.Formatting.MultipleStatementAlignment.NotSameWarning" />
         <exclude name="Generic.Functions.OpeningFunctionBraceBsdAllman.BraceOnSameLine" />
diff --git a/tests/ResponseTest.php b/tests/ResponseTest.php
index 3226cc9..443ff89 100644
--- a/tests/ResponseTest.php
+++ b/tests/ResponseTest.php
@@ -270,6 +270,22 @@ class ResponseTest extends TestCase
         $this->assertEquals('grfg', $rot13_body);
     }
 
+    public function testResponseBodyCallbackGzip()
+    {
+        $response = new Response();
+        $response->content_length = true;
+        $response->write('test');
+        $gzip = function ($body) {
+            return gzencode($body);
+        };
+        $response->addResponseBodyCallback($gzip);
+        ob_start();
+        $response->send();
+        $gzip_body = ob_get_clean();
+        $this->assertEquals('H4sIAAAAAAAAAytJLS4BAAx+f9gEAAAA', base64_encode($gzip_body));
+        $this->assertEquals(strlen(gzencode('test')), strlen($gzip_body));
+    }
+
     public function testResponseBodyCallbackMultiple()
     {
         $response = new Response();