diff --git a/subtree/rabit/Makefile b/subtree/rabit/Makefile
index 47c16fbde..b090961c6 100644
--- a/subtree/rabit/Makefile
+++ b/subtree/rabit/Makefile
@@ -1,4 +1,4 @@
-ifndef 
+ifndef CXX
 export CXX = g++
 endif
 export MPICXX = mpicxx