diff --git a/ui/vnc.c b/ui/vnc.c
index 7330b2c8894d1aedf6dcb1e6d33964658d128321..7fc40acae85a67b4568fee4de526322bb4bdbdb7 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -796,6 +796,7 @@ static int vnc_cursor_define(VncState *vs)
     int isize;
 
     if (vnc_has_feature(vs, VNC_FEATURE_RICH_CURSOR)) {
+        vnc_lock_output(vs);
         vnc_write_u8(vs,  VNC_MSG_SERVER_FRAMEBUFFER_UPDATE);
         vnc_write_u8(vs,  0);  /*  padding     */
         vnc_write_u16(vs, 1);  /*  # of rects  */
@@ -804,6 +805,7 @@ static int vnc_cursor_define(VncState *vs)
         isize = c->width * c->height * vs->clientds.pf.bytes_per_pixel;
         vnc_write_pixels_generic(vs, &pf, c->data, isize);
         vnc_write(vs, vs->vd->cursor_mask, vs->vd->cursor_msize);
+        vnc_unlock_output(vs);
         return 0;
     }
     return -1;