diff --git a/grbl/nuts_bolts.h b/grbl/nuts_bolts.h
index c936f2b..4b81805 100644
--- a/grbl/nuts_bolts.h
+++ b/grbl/nuts_bolts.h
@@ -22,8 +22,10 @@
 #ifndef nuts_bolts_h
 #define nuts_bolts_h
 
+#ifndef false
 #define false 0
 #define true 1
+#endif
 
 #define SOME_LARGE_VALUE 1.0E+38