diff --git a/src/PrepareBodyMiddleware.php b/src/PrepareBodyMiddleware.php
index 2eb95f9b2..0c9397c67 100644
--- a/src/PrepareBodyMiddleware.php
+++ b/src/PrepareBodyMiddleware.php
@@ -54,7 +54,7 @@ public function __invoke(RequestInterface $request, array $options)
         ) {
             $size = $request->getBody()->getSize();
             if ($size !== null) {
-                $modify['set_headers']['Content-Length'] = $size;
+                $modify['set_headers']['Content-Length'] = (string) $size;
             } else {
                 $modify['set_headers']['Transfer-Encoding'] = 'chunked';
             }
diff --git a/tests/PrepareBodyMiddlewareTest.php b/tests/PrepareBodyMiddlewareTest.php
index 53d2316e5..18287e416 100644
--- a/tests/PrepareBodyMiddlewareTest.php
+++ b/tests/PrepareBodyMiddlewareTest.php
@@ -34,6 +34,7 @@ function (RequestInterface $request) use ($body) {
                 $length = strlen($body);
                 if ($length > 0) {
                     $this->assertEquals($length, $request->getHeaderLine('Content-Length'));
+                    $this->assertTrue(is_string($request->getHeaderLine('Content-Length')));
                 } else {
                     $this->assertFalse($request->hasHeader('Content-Length'));
                 }