Index: libavcodec/vc1.c
===================================================================
--- libavcodec/vc1.c    (revision 7322)
+++ libavcodec/vc1.c    (working copy)
@@ -3072,8 +3072,8 @@
         ac_val -= 16 * s->block_wrap[n];
 
     q1 = s->current_picture.qscale_table[mb_pos];
-    if(dc_pred_dir && c_avail) q2 = s->current_picture.qscale_table[mb_pos - 1];
-    if(!dc_pred_dir && a_avail) q2 = s->current_picture.qscale_table[mb_pos - s->mb_stride];
+    if(dc_pred_dir && c_avail && mb_pos) q2 = s->current_picture.qscale_table[mb_pos - 1];
+    if(!dc_pred_dir && a_avail && mb_pos > s->mb_stride) q2 = s->current_picture.qscale_table[mb_pos - s->mb_stride];
     if(n && n<4) q2 = q1;
 
     if(coded) {