diff --git a/board/st/common/stm32mp_dfu.c b/board/st/common/stm32mp_dfu.c
index 1db8e45..8c1f80b 100644
--- a/board/st/common/stm32mp_dfu.c
+++ b/board/st/common/stm32mp_dfu.c
@@ -105,7 +105,7 @@
 	if (env_get("dfu_alt_info"))
 		return;
 
-	memset(buf, 0, sizeof(buf));
+	memset(buf, 0, DFU_ALT_BUF_LEN);
 
 	snprintf(buf, DFU_ALT_BUF_LEN,
 		 "ram 0=%s", CONFIG_DFU_ALT_RAM0);