diff --git a/.travis.yml b/.travis.yml
index 6702384..8c79527 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -217,7 +217,7 @@ install:
 script:
   - |-
     echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
-  - ./b2 -j3 libs/mp11/test toolset=$TOOLSET cxxstd=$CXXSTD cxxflags=$CXXFLAGS
+  - ./b2 -j3 libs/mp11/test toolset=$TOOLSET cxxstd=$CXXSTD ${CXXFLAGS:+cxxflags="$CXXFLAGS"}
 
 notifications:
   email: