diff --git a/src/ProcessLauncher.php b/src/ProcessLauncher.php
index c32f248..adeddaa 100644
--- a/src/ProcessLauncher.php
+++ b/src/ProcessLauncher.php
@@ -41,7 +41,7 @@ public function createDeferredShell($process)
         // forcefully terminate process when stream closes
         $stream->on('close', function () use ($process) {
             if ($process->isRunning()) {
-                $process->terminate(SIGKILL);
+                $process->terminate(defined('SIGKILL') ? SIGKILL : null);
             }
         });
 
diff --git a/tests/ProcessLauncherTest.php b/tests/ProcessLauncherTest.php
index 884e03d..f31a4a0 100644
--- a/tests/ProcessLauncherTest.php
+++ b/tests/ProcessLauncherTest.php
@@ -38,7 +38,7 @@ public function testClosingStreamTerminatesRunningProcess()
         $process->stdin->expects($this->any())->method('isWritable')->willReturn(true);
 
         $process->expects($this->once())->method('isRunning')->will($this->returnValue(true));
-        $process->expects($this->once())->method('terminate')->with($this->equalTo(SIGKILL));
+        $process->expects($this->once())->method('terminate')->with($this->equalTo(defined('SIGKILL') ? SIGKILL : null));
 
         $shell = $this->processLauncher->createDeferredShell($process);