diff --git a/src/ApplicationDefaultCredentials.php b/src/ApplicationDefaultCredentials.php
index c2cf66e39..2b94b4b86 100644
--- a/src/ApplicationDefaultCredentials.php
+++ b/src/ApplicationDefaultCredentials.php
@@ -366,9 +366,9 @@ private static function onGce(
      */
     public static function getDefaultLogger(): null|LoggerInterface
     {
-        $loggingFlag = getenv(self::SDK_DEBUG_FLAG);
+        $loggingFlag = (string)getenv(self::SDK_DEBUG_FLAG);
 
-        if (!$loggingFlag || strtolower(getenv((string) $loggingFlag)) !== 'true') {
+        if (!$loggingFlag || strtolower($loggingFlag) !== 'true') {
             return null;
         }
 
diff --git a/src/HttpHandler/Guzzle6HttpHandler.php b/src/HttpHandler/Guzzle6HttpHandler.php
index a47ceee15..25f952c50 100644
--- a/src/HttpHandler/Guzzle6HttpHandler.php
+++ b/src/HttpHandler/Guzzle6HttpHandler.php
@@ -21,7 +21,7 @@
 use GuzzleHttp\ClientInterface;
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
-use Psr\log\LoggerInterface;
+use Psr\Log\LoggerInterface;
 
 class Guzzle6HttpHandler
 {
@@ -76,7 +76,7 @@ public function async(RequestInterface $request, array $options = [])
             $requestEvent->method = $request->getMethod();
             $requestEvent->url = $request->getUri()->__toString();
             $requestEvent->headers = $request->getHeaders();
-            $requestEvent->payload = json_encode($request->getBody()->getContents());
+            $requestEvent->payload = json_encode($request->getBody()->getContents()) ?? null;
             $requestEvent->retryAttempt = $options['retryAttempt'] ?? null;
 
             $this->logRequest($requestEvent);
@@ -87,8 +87,8 @@ public function async(RequestInterface $request, array $options = [])
                 $responseEvent = new LogEvent($requestEvent->timestamp);
 
                 $responseEvent->headers = $response->getHeaders();
-                $responseEvent->payload = json_encode($response->getBody()->getContents());
-                $response->status = $response->getStatusCode();
+                $responseEvent->payload = json_encode($response->getBody()->getContents()) ?? null;
+                $responseEvent->status = $response->getStatusCode();
 
                 $this->logResponse($responseEvent);
             }
diff --git a/src/Logger/LoggingTrait.php b/src/Logger/LoggingTrait.php
index 83ba46509..c702cb4af 100644
--- a/src/Logger/LoggingTrait.php
+++ b/src/Logger/LoggingTrait.php
@@ -85,7 +85,7 @@ private function logResponse(LogEvent $event): void
 
     /**
      * @param array<string> $headers
-     * @return null|array<string>
+     * @return null|array<string,string>
      */
     private function getJwtToken(array $headers): null|array
     {