diff --git a/composer.json b/composer.json
index a20c501..06e6a2a 100644
--- a/composer.json
+++ b/composer.json
@@ -3,7 +3,7 @@
     "homepage": "https://github.com/amphp/process",
     "description": "Asynchronous process manager.",
     "require": {
-        "php": ">=7",
+        "php": ">=7.1",
         "amphp/amp": "^2",
         "amphp/byte-stream": "^1.4"
     },
@@ -40,7 +40,7 @@
     },
     "config": {
         "platform": {
-            "php": "7.0.33"
+            "php": "7.1.3"
         }
     },
     "scripts": {
diff --git a/lib/Internal/Posix/Runner.php b/lib/Internal/Posix/Runner.php
index dffbc4d..d073b79 100644
--- a/lib/Internal/Posix/Runner.php
+++ b/lib/Internal/Posix/Runner.php
@@ -78,7 +78,7 @@ public static function onProcessStartExtraDataPipeReadable($watcher, $stream, $d
     }
 
     /** @inheritdoc */
-    public function start(string $command, string $cwd = null, array $env = [], array $options = []): ProcessHandle
+    public function start(string $command, ?string $cwd = null, array $env = [], array $options = []): ProcessHandle
     {
         $command = \sprintf(
             '{ (%s) <&3 3<&- 3>/dev/null & } 3<&0; trap "" INT TERM QUIT HUP;' .
diff --git a/lib/Internal/ProcessRunner.php b/lib/Internal/ProcessRunner.php
index 3adf0f2..6c12821 100644
--- a/lib/Internal/ProcessRunner.php
+++ b/lib/Internal/ProcessRunner.php
@@ -19,7 +19,7 @@ interface ProcessRunner
      *
      * @throws ProcessException If starting the process fails.
      */
-    public function start(string $command, string $cwd = null, array $env = [], array $options = []): ProcessHandle;
+    public function start(string $command, ?string $cwd = null, array $env = [], array $options = []): ProcessHandle;
 
     /**
      * Wait for the child process to end.
diff --git a/lib/Internal/Windows/Runner.php b/lib/Internal/Windows/Runner.php
index d4bbd86..6bca96c 100644
--- a/lib/Internal/Windows/Runner.php
+++ b/lib/Internal/Windows/Runner.php
@@ -76,7 +76,7 @@ public function __construct()
     }
 
     /** @inheritdoc */
-    public function start(string $command, string $cwd = null, array $env = [], array $options = []): ProcessHandle
+    public function start(string $command, ?string $cwd = null, array $env = [], array $options = []): ProcessHandle
     {
         if (\strpos($command, "\0") !== false) {
             throw new ProcessException("Can't execute commands that contain null bytes.");
diff --git a/lib/Process.php b/lib/Process.php
index 2826b80..5c7c40c 100644
--- a/lib/Process.php
+++ b/lib/Process.php
@@ -43,7 +43,7 @@ final class Process
      *
      * @throws \Error If the arguments are invalid.
      */
-    public function __construct($command, string $cwd = null, array $env = [], array $options = [])
+    public function __construct($command, ?string $cwd = null, array $env = [], array $options = [])
     {
         $command = \is_array($command)
             ? \implode(" ", \array_map(__NAMESPACE__ . "\\escapeArguments", $command))