diff --git a/spec/Plugin/ContentLengthPluginSpec.php b/spec/Plugin/ContentLengthPluginSpec.php
index 75e913e..4ec2ba7 100644
--- a/spec/Plugin/ContentLengthPluginSpec.php
+++ b/spec/Plugin/ContentLengthPluginSpec.php
@@ -25,7 +25,7 @@ function it_adds_content_length_header(RequestInterface $request, StreamInterfac
         $request->hasHeader('Content-Length')->shouldBeCalled()->willReturn(false);
         $request->getBody()->shouldBeCalled()->willReturn($stream);
         $stream->getSize()->shouldBeCalled()->willReturn(100);
-        $request->withHeader('Content-Length', 100)->shouldBeCalled()->willReturn($request);
+        $request->withHeader('Content-Length', '100')->shouldBeCalled()->willReturn($request);
 
         $this->handleRequest($request, function () {}, function () {});
     }
diff --git a/src/Plugin/ContentLengthPlugin.php b/src/Plugin/ContentLengthPlugin.php
index a740c19..0f7aafa 100644
--- a/src/Plugin/ContentLengthPlugin.php
+++ b/src/Plugin/ContentLengthPlugin.php
@@ -27,7 +27,7 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
                 $request = $request->withBody($stream);
                 $request = $request->withAddedHeader('Transfer-Encoding', 'chunked');
             } else {
-                $request = $request->withHeader('Content-Length', $stream->getSize());
+                $request = $request->withHeader('Content-Length', (string) $stream->getSize());
             }
         }