diff --git a/CMakeLists.txt b/CMakeLists.txt
index 00ea485..ca8ad57 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -119,7 +119,8 @@ if (NOT LIBCXX_ENABLE_RTTI)
   append_if(LIBCXX_CXX_FEATURE_FLAGS LIBCXX_HAS_FNO_RTTI_FLAG -fno-rtti)
 endif()
 # Assert
-if (LLVM_ENABLE_ASSERTIONS)
+string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
+if (LIBCXX_ENABLE_ASSERTIONS)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
   if (NOT MSVC)
     list(APPEND LIBCXX_CXX_FEATURE_FLAGS -D_DEBUG)