diff --git a/drivers/rtc/rtc-bq32k.c b/drivers/rtc/rtc-bq32k.c
index e8698e9870fe..ef52741000a8 100644
--- a/drivers/rtc/rtc-bq32k.c
+++ b/drivers/rtc/rtc-bq32k.c
@@ -36,6 +36,10 @@
 #define BQ32K_CFG2		0x09	/* Trickle charger control */
 #define BQ32K_TCFE		BIT(6)	/* Trickle charge FET bypass */
 
+#define MAX_LEN			10	/* Maximum number of consecutive
+					 * register for this particular RTC.
+					 */
+
 struct bq32k_regs {
 	uint8_t		seconds;
 	uint8_t		minutes;
@@ -74,7 +78,7 @@ static int bq32k_read(struct device *dev, void *data, uint8_t off, uint8_t len)
 static int bq32k_write(struct device *dev, void *data, uint8_t off, uint8_t len)
 {
 	struct i2c_client *client = to_i2c_client(dev);
-	uint8_t buffer[len + 1];
+	uint8_t buffer[MAX_LEN + 1];
 
 	buffer[0] = off;
 	memcpy(&buffer[1], data, len);
diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
index 79e24eadbe99..00e11c1b2186 100644
--- a/drivers/rtc/rtc-mcp795.c
+++ b/drivers/rtc/rtc-mcp795.c
@@ -82,7 +82,7 @@ static int mcp795_rtcc_write(struct device *dev, u8 addr, u8 *data, u8 count)
 {
 	struct spi_device *spi = to_spi_device(dev);
 	int ret;
-	u8 tx[2 + count];
+	u8 tx[257];
 
 	tx[0] = MCP795_WRITE;
 	tx[1] = addr;