diff --git a/kitty/mouse.c b/kitty/mouse.c
index 766767313..c9cef78ec 100644
--- a/kitty/mouse.c
+++ b/kitty/mouse.c
@@ -66,8 +66,8 @@ encode_mouse_event(Window *w, int button, MouseAction action, int mods) {
             return snprintf(mouse_event_buf, sizeof(mouse_event_buf), "\033[%d;%d;%dM", cb, x, y);
             break;
         case UTF8_PROTOCOL:
-            mouse_event_buf[0] = 033; mouse_event_buf[1] = '['; mouse_event_buf[2] = cb + 32; 
-            unsigned int sz = 3;
+            mouse_event_buf[0] = 033; mouse_event_buf[1] = '['; mouse_event_buf[2] = 'M'; mouse_event_buf[3] = cb + 32; 
+            unsigned int sz = 4;
             sz += encode_utf8(x + 32, mouse_event_buf + sz);
             sz += encode_utf8(y + 32, mouse_event_buf + sz);
             return sz;