diff --git a/gtsam/base/timing.h b/gtsam/base/timing.h
index ecec413b1..030754cde 100644
--- a/gtsam/base/timing.h
+++ b/gtsam/base/timing.h
@@ -26,7 +26,7 @@
 
 // Automatically use the new Boost timers if version is recent enough.
 #if BOOST_VERSION >= 104800
-#if not GTSAM_DISABLE_NEW_TIMERS
+#ifndef GTSAM_DISABLE_NEW_TIMERS
 #define GTSAM_USING_NEW_BOOST_TIMERS
 #endif
 #endif