diff --git a/third_party/mbedtls/Makefile.am b/third_party/mbedtls/Makefile.am
index 8addb9583..917ef46ae 100644
--- a/third_party/mbedtls/Makefile.am
+++ b/third_party/mbedtls/Makefile.am
@@ -24,8 +24,8 @@ EXTRA_DIST                                    = \
 lib_LIBRARIES                                 = libmbedcrypto.a
 
 # Do not enable -Wconversion for mbedtls
-CPPFLAGS                                     := $(filter-out -Wconversion,$(CPPFLAGS))
-CFLAGS                                       := $(filter-out -Wconversion,$(CFLAGS))
+override CFLAGS                              := $(filter-out -Wconversion,$(CFLAGS))
+override CXXFLAGS                            := $(filter-out -Wconversion,$(CXXFLAGS))
 
 MBEDTLS_SRCDIR                                = $(top_srcdir)/third_party/mbedtls/repo