diff --git a/package/mbedtls/mbedtls.mk b/package/mbedtls/mbedtls.mk
index 5facb99b2a..8745ff33f4 100644
--- a/package/mbedtls/mbedtls.mk
+++ b/package/mbedtls/mbedtls.mk
@@ -7,6 +7,7 @@
 MBEDTLS_VERSION = 2.28.0
 MBEDTLS_SITE = $(call github,ARMmbed,mbedtls,v$(MBEDTLS_VERSION))
 MBEDTLS_CONF_OPTS = \
+	-DCMAKE_C_FLAGS="$(TARGET_CFLAGS) -std=c99" \
 	-DENABLE_PROGRAMS=$(if $(BR2_PACKAGE_MBEDTLS_PROGRAMS),ON,OFF) \
 	-DENABLE_TESTING=OFF \
 	-DMBEDTLS_FATAL_WARNINGS=OFF