diff --git a/composer.json b/composer.json
index 3abddac..756b44d 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,7 @@
 	],
 	"require": {
 		"php": "^8.4",
-		"guzzlehttp/guzzle": "^7.15.2",
+		"guzzlehttp/guzzle": "^8.0.1",
 		"guzzlehttp/uri-template": "^1.0.10"
 	},
 	"require-dev": {
diff --git a/src/Http/JsonBody.php b/src/Http/JsonBody.php
index b4f84d2..6731ceb 100644
--- a/src/Http/JsonBody.php
+++ b/src/Http/JsonBody.php
@@ -45,7 +45,7 @@ public function getContentType()
      */
     public function getContentLength()
     {
-        return strlen($this->getContent());
+        return (string) strlen($this->getContent());
     }
 
     /**
diff --git a/src/Http/UrlEncodedBody.php b/src/Http/UrlEncodedBody.php
index b7e26f7..86afe74 100644
--- a/src/Http/UrlEncodedBody.php
+++ b/src/Http/UrlEncodedBody.php
@@ -45,7 +45,7 @@ public function getContentType()
      */
     public function getContentLength()
     {
-        return strlen($this->getContent());
+        return (string) strlen($this->getContent());
     }
 
     /**