diff --git a/ext/tpx/RedlichKwong.h b/ext/tpx/RedlichKwong.h
index 5861e60df..e7361f91b 100644
--- a/ext/tpx/RedlichKwong.h
+++ b/ext/tpx/RedlichKwong.h
@@ -2,6 +2,7 @@
 #define TPX_RK_H
 
 #include "Sub.h"
+#include <math.h>
 
 namespace tpx {