diff --git a/boot/zephyr/include/config-kw.h b/boot/zephyr/include/config-kw.h
index 5eeab50..41e5baa 100644
--- a/boot/zephyr/include/config-kw.h
+++ b/boot/zephyr/include/config-kw.h
@@ -46,6 +46,7 @@
 
 #define MBEDTLS_PLATFORM_EXIT_ALT
 #define MBEDTLS_PLATFORM_PRINTF_ALT
+#define MBEDTLS_PLATFORM_SNPRINTF_ALT
 
 #define MBEDTLS_ASN1_PARSE_C