diff --git a/src/device/mouse_ps2.c b/src/device/mouse_ps2.c
index f623d23e9..2d0f3c85d 100644
--- a/src/device/mouse_ps2.c
+++ b/src/device/mouse_ps2.c
@@ -86,7 +86,7 @@ ps2_report_coordinates(atkbc_dev_t *dev, int main)
                           -256, 255, 1, 0);
     mouse_subtract_z(&delta_z, -8, 7, 1);
 
-    buff[0] = (overflow_y << 7) | (overflow_x << 6) |
+    buff[0] |= (overflow_y << 7) | (overflow_x << 6) |
               ((delta_y & 0x0100) >> 3) | ((delta_x & 0x0100) >> 4) |
               (b & ((dev->flags & FLAG_INTELLI) ? 0x07 : 0x03));
     buff[1] = (delta_x & 0x00ff);