diff --git a/components/drivers/ofw/fdt.c b/components/drivers/ofw/fdt.c
index cca4cdd20d5..cfd6c32f23f 100644
--- a/components/drivers/ofw/fdt.c
+++ b/components/drivers/ofw/fdt.c
@@ -593,7 +593,7 @@ rt_err_t rt_fdt_scan_chosen_stdout(void)
     int len, options_len = 0;
     const char *options = RT_NULL, *con_type = RT_NULL;
 
-    rt_memset(&fdt_earlycon, 0, sizeof(fdt_earlycon) - sizeof(fdt_earlycon.msg));
+    rt_memset(&fdt_earlycon, 0, rt_offsetof(struct rt_fdt_earlycon, msg_idx));
     fdt_earlycon.nodeoffset = -1;
 
     offset = fdt_path_offset(_fdt, "/chosen");