diff --git a/hw/input/ps2.c b/hw/input/ps2.c
index 5352e417a4..7a3fb2b9f6 100644
--- a/hw/input/ps2.c
+++ b/hw/input/ps2.c
@@ -646,7 +646,8 @@ void ps2_keyboard_set_translation(void *opaque, int mode)
 
 static int ps2_mouse_send_packet(PS2MouseState *s)
 {
-    const int needed = 3 + (s->mouse_type - 2);
+    /* IMPS/2 and IMEX send 4 bytes, PS2 sends 3 bytes */
+    const int needed = s->mouse_type ? 4 : 3;
     unsigned int b;
     int dx1, dy1, dz1;