diff --git a/cmake/flags.cmake b/cmake/flags.cmake
index d8a38890338032957bda2618f9676d938f864184..7eb92efcb00fa18461e61e0508b485c13ef23a1f 100644
--- a/cmake/flags.cmake
+++ b/cmake/flags.cmake
@@ -26,7 +26,7 @@ function(CheckCompilerCXX11Flag)
 endfunction()
 
 CheckCompilerCXX11Flag()
-string(APPEND CMAKE_CXX_FLAGS " -std=c++11")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 
 # safe_set_flag
 #