diff --git a/Core/src/Upload/MultipartUploader.php b/Core/src/Upload/MultipartUploader.php
index 2b0b52e717bb..06522f2e7075 100644
--- a/Core/src/Upload/MultipartUploader.php
+++ b/Core/src/Upload/MultipartUploader.php
@@ -95,7 +95,7 @@ private function prepareRequest()
 
         $size = $multipartStream->getSize();
         if ($size !== null) {
-            $headers['Content-Length'] = $size;
+            $headers['Content-Length'] = (string) $size;
         }
 
         $customHeaders = $this->requestOptions['restOptions']['headers'] ?? [];
diff --git a/Core/src/Upload/ResumableUploader.php b/Core/src/Upload/ResumableUploader.php
index 1fb9c810a8cb..f970dde52c7e 100644
--- a/Core/src/Upload/ResumableUploader.php
+++ b/Core/src/Upload/ResumableUploader.php
@@ -165,7 +165,7 @@ public function upload()
             $rangeEnd = $rangeStart + ($currStreamLimitSize - 1);
 
             $headers = $this->headers + [
-                'Content-Length' => $currStreamLimitSize,
+                'Content-Length' => (string) $currStreamLimitSize,
                 'Content-Type' => $this->contentType,
                 'Content-Range' => "bytes $rangeStart-$rangeEnd/$size",
             ];
@@ -245,7 +245,7 @@ protected function createResumeUri()
     {
         $headers = $this->headers + [
             'X-Upload-Content-Type' => $this->contentType,
-            'X-Upload-Content-Length' => $this->data->getSize(),
+            'X-Upload-Content-Length' => (string) $this->data->getSize(),
             'Content-Type' => 'application/json'
         ];
 
diff --git a/Core/src/Upload/SignedUrlUploader.php b/Core/src/Upload/SignedUrlUploader.php
index 4a7d8aa77bbd..8408a855f3c3 100644
--- a/Core/src/Upload/SignedUrlUploader.php
+++ b/Core/src/Upload/SignedUrlUploader.php
@@ -71,7 +71,7 @@ protected function createResumeUri()
     {
         $headers = $this->headers + [
             'Content-Type' => $this->contentType,
-            'Content-Length' => 0,
+            'Content-Length' => '0',
             'x-goog-resumable' => 'start'
         ];
 
diff --git a/Core/src/Upload/StreamableUploader.php b/Core/src/Upload/StreamableUploader.php
index d3221488309c..8aafac756471 100644
--- a/Core/src/Upload/StreamableUploader.php
+++ b/Core/src/Upload/StreamableUploader.php
@@ -59,7 +59,7 @@ public function upload($writeSize = null)
 
         // do the streaming write
         $headers = [
-            'Content-Length'    => $writeSize,
+            'Content-Length'    => (string) $writeSize,
             'Content-Type'      => $this->contentType,
             'Content-Range'     => "bytes {$this->rangeStart}-$rangeEnd/*"
         ];
diff --git a/Core/tests/Unit/Upload/MultipartUploaderTest.php b/Core/tests/Unit/Upload/MultipartUploaderTest.php
index 91c30a1d654f..89f30d942706 100644
--- a/Core/tests/Unit/Upload/MultipartUploaderTest.php
+++ b/Core/tests/Unit/Upload/MultipartUploaderTest.php
@@ -137,6 +137,11 @@ public function testStreamWithoutSizeDoesNotReceiveContentLengthHeader($stream,
             Argument::that(
                 function (RequestInterface $request) use ($shouldContentLengthExist) {
                     $this->assertEquals($request->hasHeader('Content-Length'), $shouldContentLengthExist);
+
+                    foreach ($request->getHeader('Content-Length') as $value) {
+                        $this->assertIsString($value);
+                    }
+
                     return true;
                 }
             ),
diff --git a/Core/tests/Unit/Upload/ResumableUploaderTest.php b/Core/tests/Unit/Upload/ResumableUploaderTest.php
index 2ab311d396b0..d2b2235926e6 100644
--- a/Core/tests/Unit/Upload/ResumableUploaderTest.php
+++ b/Core/tests/Unit/Upload/ResumableUploaderTest.php
@@ -111,7 +111,9 @@ public function testGetResumeUri()
         $response = new Response(200, ['Location' => $resumeUri]);
 
         $this->requestWrapper->send(
-            Argument::type(RequestInterface::class),
+            Argument::that(function (RequestInterface $request) {
+                return $request->getHeader('X-Upload-Content-Length') === ['4'];
+            }),
             Argument::type('array')
         )->willReturn($response);
 
@@ -282,6 +284,7 @@ public function testUploadDoesNotSendGoogHashOnIntermediateChunk()
         $this->requestWrapper->send(
             Argument::that(function (RequestInterface $request) {
                 return $request->getHeaderLine('Content-Range') === 'bytes 0-1/4'
+                    && $request->getHeader('Content-Length') === ['2']
                     && !$request->hasHeader('X-Goog-Hash');
             }),
             Argument::type('array')
diff --git a/Core/tests/Unit/Upload/SignedUrlUploaderTest.php b/Core/tests/Unit/Upload/SignedUrlUploaderTest.php
index 8188a8c84553..b7c376037b3d 100644
--- a/Core/tests/Unit/Upload/SignedUrlUploaderTest.php
+++ b/Core/tests/Unit/Upload/SignedUrlUploaderTest.php
@@ -62,7 +62,7 @@ public function testGetResumeUri()
                     return false;
                 }
 
-                if ($arg->getHeaderLine('Content-Length') != 0) {
+                if ($arg->getHeader('Content-Length') !== ['0']) {
                     return false;
                 }
 
diff --git a/Core/tests/Unit/Upload/StreamableUploaderTest.php b/Core/tests/Unit/Upload/StreamableUploaderTest.php
index 2dfeb0422db2..79434c7376c0 100644
--- a/Core/tests/Unit/Upload/StreamableUploaderTest.php
+++ b/Core/tests/Unit/Upload/StreamableUploaderTest.php
@@ -160,7 +160,7 @@ public function testLastChunkSendsCorrectHeaders()
 
         $this->requestWrapper->send(
             Argument::that(function ($request) {
-                return $request->getHeaderLine('Content-Length') == '10';
+                return $request->getHeader('Content-Length') === ['10'];
             }),
             Argument::type('array')
         )->willReturn($response)->shouldBeCalled();