diff --git a/sandboxed_api/sandbox2/monitor.cc b/sandboxed_api/sandbox2/monitor.cc
index 045d7a4..8d5242b 100644
--- a/sandboxed_api/sandbox2/monitor.cc
+++ b/sandboxed_api/sandbox2/monitor.cc
@@ -950,7 +950,7 @@ void Monitor::StateProcessStopped(pid_t pid, int status) {
     should_dump_stack_ = false;
   }
 
-#ifndef PTRACE_EVENT_STOP
+#if !defined(PTRACE_EVENT_STOP)
 #define PTRACE_EVENT_STOP 128
 #endif