diff --git a/src/ProcessBase.php b/src/ProcessBase.php
index 2ad6186..c071ee8 100644
--- a/src/ProcessBase.php
+++ b/src/ProcessBase.php
@@ -125,7 +125,7 @@ public function setLogger($logger)
     /**
      * @inheritDoc
      */
-    public function start(callable $callback = null, array $env = []): void
+    public function start(?callable $callback = null, array $env = []): void
     {
         $cmd = $this->getCommandLine();
         if ($this->isSimulated()) {
diff --git a/src/ProcessManager.php b/src/ProcessManager.php
index acfe12b..8476731 100644
--- a/src/ProcessManager.php
+++ b/src/ProcessManager.php
@@ -103,7 +103,7 @@ public function siteProcess(SiteAliasInterface $siteAlias, $args = [], $options
      * @param int|float|null $timeout The timeout in seconds or null to disable
      * @return Process
      */
-    public function process($command, $cwd = null, array $env = null, $input = null, $timeout = 60)
+    public function process($command, $cwd = null, ?array $env = null, $input = null, $timeout = 60)
     {
         return new ProcessBase($command, $cwd, $env, $input, $timeout);
     }
@@ -117,7 +117,7 @@ public function process($command, $cwd = null, array $env = null, $input = null,
      * @param int|float|null $timeout The timeout in seconds or null to disable
      * @return Process
      */
-    public function shell($command, $cwd = null, array $env = null, $input = null, $timeout = 60)
+    public function shell($command, $cwd = null, ?array $env = null, $input = null, $timeout = 60)
     {
         return ProcessBase::fromShellCommandline($command, $cwd, $env, $input, $timeout);
     }
diff --git a/src/SiteProcess.php b/src/SiteProcess.php
index c0aec48..8abf65b 100644
--- a/src/SiteProcess.php
+++ b/src/SiteProcess.php
@@ -198,13 +198,13 @@ public function getCommandLine(): string
     /**
      * @inheritDoc
      */
-    public function start(callable $callback = null, array $env = []): void
+    public function start(?callable $callback = null, array $env = []): void
     {
         $cmd = $this->getCommandLine();
         parent::start($callback, $env);
     }
 
-    public function mustRun(callable $callback = null, array $env = []): static
+    public function mustRun(?callable $callback = null, array $env = []): static
     {
         if (0 !== $this->run($callback, $env)) {
             // Be less verbose when there is nothing in stdout or stderr.
@@ -220,7 +220,7 @@ public function mustRun(callable $callback = null, array $env = []): static
     /**
      * @inheritDoc
      */
-    public function wait(callable $callback = null): int
+    public function wait(?callable $callback = null): int
     {
         $return = parent::wait($callback);
         return $return;