diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0c1813e..018fdd1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -82,7 +82,7 @@ elseif(${IS_CLANG})
     endif()
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fstrict-vtable-pointers")
 endif()
-string(TOUPPER ${CMAKE_BUILD_TYPE} UPPERCASE_BUILD_TYPE)
+string(TOUPPER "${CMAKE_BUILD_TYPE}" UPPERCASE_BUILD_TYPE)
 if (UPPERCASE_BUILD_TYPE STREQUAL RELEASE)
     add_definitions(-DYBCON_FLAGS="${CMAKE_CXX_FLAGS_RELEASE}")
 endif()