diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -295,11 +295,11 @@
 
   # 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")
     add_compile_flags(-UNDEBUG)
   endif()
 else()
-  if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "RELEASE")
+  if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     add_compile_flags(-DNDEBUG)
   endif()
 endif()