diff --git a/boot/zephyr/include/config-rsa.h b/boot/zephyr/include/config-rsa.h
index f6bba7e..2e1153d 100644
--- a/boot/zephyr/include/config-rsa.h
+++ b/boot/zephyr/include/config-rsa.h
@@ -46,6 +46,7 @@
 
 #define MBEDTLS_PLATFORM_EXIT_ALT
 #define MBEDTLS_PLATFORM_PRINTF_ALT
+#define MBEDTLS_PLATFORM_SNPRINTF_ALT
 
 #if !defined(CONFIG_ARM)
 #define MBEDTLS_HAVE_ASM