diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -342,11 +342,11 @@
   endif()
   # On Release builds cmake automatically defines NDEBUG, so we
   # explicitly undefine it:
-  if (uppercase_CMAKE_BUILD_TYPE STREQUAL "RELEASE")
+  if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     list(APPEND LIBCXXABI_COMPILE_FLAGS -UNDEBUG)
   endif()
 else()
-  if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "RELEASE")
+  if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     list(APPEND LIBCXXABI_COMPILE_FLAGS -DNDEBUG)
   endif()
 endif()