diff --git a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
index 55414dc75fbf6..c094a64c7a79f 100644
--- a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
+++ b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
@@ -40,6 +40,7 @@ if (current_toolchain == default_toolchain) {
       "_LIBCPP_HAS_NO_WIDE_CHARACTERS=",
       "_LIBCPP_ABI_DEFINES=",
       "_LIBCPP_ENABLE_ASSERTIONS_DEFAULT=",
+      "_LIBCPP_ENABLE_DEBUG_MODE=",
     ]
     if (libcxx_abi_version != 1) {
       values += [ "_LIBCPP_ABI_VERSION=$libcxx_abi_version" ]