diff --git a/src/Zen/BaseZen.php b/src/Zen/BaseZen.php
index 104a002..552ca13 100644
--- a/src/Zen/BaseZen.php
+++ b/src/Zen/BaseZen.php
@@ -55,7 +55,7 @@ public function close()
         $this->deferred->resolve();
 
         if ($this->process !== null && $this->process->isRunning()) {
-            $this->process->terminate(SIGKILL);
+            $this->process->terminate(defined('SIGKILL') ? SIGKILL : null);
 
             // explicitly close all streams immediately
             $this->process->stdin->close();
diff --git a/tests/Zen/FunctionalBaseZenTest.php b/tests/Zen/FunctionalBaseZenTest.php
index ba2aa3d..1242d25 100644
--- a/tests/Zen/FunctionalBaseZenTest.php
+++ b/tests/Zen/FunctionalBaseZenTest.php
@@ -76,7 +76,7 @@ public function testTerminatingProcessReturnsError()
         $zen->go($process);
 
         $this->loop->addTimer(0.1, function() use ($process) {
-            $process->terminate(SIGKILL);
+            $process->terminate(defined('SIGKILL') ? SIGKILL : null);
             $process->stdin->end();
         });