diff --git a/knx_iot_virtual_pb.c b/knx_iot_virtual_pb.c
index 8bb594b..07c0b40 100644
--- a/knx_iot_virtual_pb.c
+++ b/knx_iot_virtual_pb.c
@@ -432,7 +432,7 @@ app_init(void)
   /* set the firmware version 0.7.0 */
   oc_core_set_device_fwv(0, 0, 7, 0);
 
-  char mid[20];
+  char mid[5];
   strncpy(mid, g_serial_number, 5); // mid = first 4 digits of sn
   mid[4] = '\0';
   long int mid_num = strtol(mid, NULL, 16);
diff --git a/knx_iot_virtual_sa.c b/knx_iot_virtual_sa.c
index 1905061..6638023 100644
--- a/knx_iot_virtual_sa.c
+++ b/knx_iot_virtual_sa.c
@@ -476,7 +476,7 @@ app_init(void)
   /* set the firmware version 0.7.0 */
   oc_core_set_device_fwv(0, 0, 7, 0);
 
-  char mid[20];
+  char mid[5];
   strncpy(mid, g_serial_number, 5); // mid = first 4 digits of sn
   mid[4] = '\0';
   long int mid_num = strtol(mid, NULL, 16);