diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index 379910db2d..0f05e4596e 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -661,9 +661,14 @@ static void cirrus_invalidate_region(CirrusVGAState * s, int off_begin,
     int off_cur;
     int off_cur_end;
 
+    if (off_pitch < 0) {
+        off_begin -= bytesperline - 1;
+    }
+
     for (y = 0; y < lines; y++) {
 	off_cur = off_begin;
 	off_cur_end = (off_cur + bytesperline) & s->cirrus_addr_mask;
+        assert(off_cur_end >= off_cur);
         memory_region_set_dirty(&s->vga.vram, off_cur, off_cur_end - off_cur);
 	off_begin += off_pitch;
     }