diff --git a/CMakeLists.txt b/CMakeLists.txt
index 01fc6de..47ffc10 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -80,7 +80,7 @@
 # Also inherit the CMAKE build type flags from one of the following variables depending on
 # the configured build type:
 # CMAKE_C_FLAGS_DEBUG, CMAKE_C_FLAGS_RELEASE, CMAKE_C_FLAGS_RELWITHDEBINFO, CMAKE_C_FLAGS_MINSIZEREL
-string(TOUPPER CMAKE_BUILD_TYPE build_type)
+string(TOUPPER "${CMAKE_BUILD_TYPE}" build_type)
 set(cmake_built_type_config_name CMAKE_C_FLAGS_${build_type})
 separate_arguments(cmake_c_flags_sep NATIVE_COMMAND "${CMAKE_C_FLAGS};${${cmake_built_type_config_name}}")