diff --git a/libavcodec/vc1_block.c b/libavcodec/vc1_block.c
index ab7f7a3518..38de670f46 100644
--- a/libavcodec/vc1_block.c
+++ b/libavcodec/vc1_block.c
@@ -1648,8 +1648,9 @@ static int vc1_decode_p_mb_intfr(VC1Context *v)
                 s->current_picture.motion_val[1][s->block_index[i]][0] = 0;
                 s->current_picture.motion_val[1][s->block_index[i]][1] = 0;
             }
-            s->current_picture.mb_type[mb_pos]                     = MB_TYPE_INTRA;
-            s->mb_intra = v->is_intra[s->mb_x] = 1;
+            v->is_intra[s->mb_x] = 0x3f; // Set the bitfield to all 1.
+            s->mb_intra          = 1;
+            s->current_picture.mb_type[mb_pos] = MB_TYPE_INTRA;
             fieldtx = v->fieldtx_plane[mb_pos] = get_bits1(gb);
             mb_has_coeffs = get_bits1(gb);
             if (mb_has_coeffs)
@@ -1816,7 +1817,8 @@ static int vc1_decode_p_mb_intfi(VC1Context *v)
 
     idx_mbmode = get_vlc2(gb, v->mbmode_vlc->table, VC1_IF_MBMODE_VLC_BITS, 2);
     if (idx_mbmode <= 1) { // intra MB
-        s->mb_intra = v->is_intra[s->mb_x] = 1;
+        v->is_intra[s->mb_x] = 0x3f; // Set the bitfield to all 1.
+        s->mb_intra          = 1;
         s->current_picture.motion_val[1][s->block_index[0] + v->blocks_off][0] = 0;
         s->current_picture.motion_val[1][s->block_index[0] + v->blocks_off][1] = 0;
         s->current_picture.mb_type[mb_pos + v->mb_off] = MB_TYPE_INTRA;
@@ -2093,7 +2095,8 @@ static void vc1_decode_b_mb_intfi(VC1Context *v)
 
     idx_mbmode = get_vlc2(gb, v->mbmode_vlc->table, VC1_IF_MBMODE_VLC_BITS, 2);
     if (idx_mbmode <= 1) { // intra MB
-        s->mb_intra = v->is_intra[s->mb_x] = 1;
+        v->is_intra[s->mb_x] = 0x3f; // Set the bitfield to all 1.
+        s->mb_intra          = 1;
         s->current_picture.motion_val[1][s->block_index[0]][0] = 0;
         s->current_picture.motion_val[1][s->block_index[0]][1] = 0;
         s->current_picture.mb_type[mb_pos + v->mb_off]         = MB_TYPE_INTRA;
@@ -2311,8 +2314,9 @@ static int vc1_decode_b_mb_intfr(VC1Context *v)
             s->mv[1][i][0] = s->current_picture.motion_val[1][s->block_index[i]][0] = 0;
             s->mv[1][i][1] = s->current_picture.motion_val[1][s->block_index[i]][1] = 0;
         }
+        v->is_intra[s->mb_x] = 0x3f; // Set the bitfield to all 1.
+        s->mb_intra          = 1;
         s->current_picture.mb_type[mb_pos] = MB_TYPE_INTRA;
-        s->mb_intra = v->is_intra[s->mb_x] = 1;
         fieldtx = v->fieldtx_plane[mb_pos] = get_bits1(gb);
         mb_has_coeffs = get_bits1(gb);
         if (mb_has_coeffs)