diff --git a/src/MasterSupervisor.php b/src/MasterSupervisor.php
index 03aa30ad..46ce8b01 100644
--- a/src/MasterSupervisor.php
+++ b/src/MasterSupervisor.php
@@ -69,7 +69,7 @@ class MasterSupervisor implements Pausable, Restartable, Terminable
      * @param  string  $environment
      * @return void
      */
-    public function __construct(string $environment = null)
+    public function __construct(?string $environment = null)
     {
         $this->environment = $environment;
 
diff --git a/src/ProcessPool.php b/src/ProcessPool.php
index 11697aee..6231897f 100644
--- a/src/ProcessPool.php
+++ b/src/ProcessPool.php
@@ -51,7 +51,7 @@ class ProcessPool implements Countable
      * @param  \Closure|null  $output
      * @return void
      */
-    public function __construct(SupervisorOptions $options, Closure $output = null)
+    public function __construct(SupervisorOptions $options, ?Closure $output = null)
     {
         $this->options = $options;
 
diff --git a/src/SupervisorProcess.php b/src/SupervisorProcess.php
index 48d9d330..c74b5792 100644
--- a/src/SupervisorProcess.php
+++ b/src/SupervisorProcess.php
@@ -50,7 +50,7 @@ class SupervisorProcess extends WorkerProcess
      * @param  \Closure|null  $output
      * @return void
      */
-    public function __construct(SupervisorOptions $options, $process, Closure $output = null)
+    public function __construct(SupervisorOptions $options, $process, ?Closure $output = null)
     {
         $this->options = $options;
         $this->name = $options->name;