diff --git a/src/device/mouse_serial.c b/src/device/mouse_serial.c
index 91fdd498a..c135f28ec 100644
--- a/src/device/mouse_serial.c
+++ b/src/device/mouse_serial.c
@@ -193,8 +193,8 @@ sermouse_report_msystems(mouse_t *dev)
     dev->buf[0] |= (b & 0x02) ? 0x00 : 0x01; /* right button */
     dev->buf[1] = delta_x;
     dev->buf[2] = delta_y;
-    dev->buf[2] = delta_x;    /* same as byte 1 */
-    dev->buf[3] = delta_y;    /* same as byte 2 */
+    dev->buf[3] = delta_x;    /* same as byte 1 */
+    dev->buf[4] = delta_y;    /* same as byte 2 */
 
     return 5;
 }
@@ -215,8 +215,6 @@ sermouse_report_3bp(mouse_t *dev)
     dev->buf[0] |= (b & 0x02) ? 0x01 : 0x00; /* right button */
     dev->buf[1] = delta_x;
     dev->buf[2] = delta_y;
-    dev->buf[2] = delta_x;    /* same as byte 1 */
-    dev->buf[3] = delta_y;    /* same as byte 2 */
 
     return 3;
 }