diff --git a/vnc.c b/vnc.c
index fcce00eddce951bba381a7778e798fce70231d51..b20fe21d7d48f27e803c3af00ef0d56f2d9ff5d2 100644
--- a/vnc.c
+++ b/vnc.c
@@ -258,6 +258,13 @@ static void vnc_dpy_update(DisplayState *ds, int x, int y, int w, int h)
 
     h += y;
 
+    /* round x down to ensure the loop only spans one 16-pixel block per,
+       iteration.  otherwise, if (x % 16) != 0, the last iteration may span
+       two 16-pixel blocks but we only mark the first as dirty
+    */
+    w += (x % 16);
+    x -= (x % 16);
+
     for (; y < h; y++)
 	for (i = 0; i < w; i += 16)
 	    vnc_set_bit(vs->dirty_row[y], (x + i) / 16);