diff --git a/libavcodec/vc1_block.c b/libavcodec/vc1_block.c
index b06ee9f..5b38183 100644
--- a/libavcodec/vc1_block.c
+++ b/libavcodec/vc1_block.c
@@ -188,7 +188,7 @@ static void vc1_put_signed_blocks_clamped(VC1Context *v)
             mquant = v->altpq;                                 \
         if ((edges&8) && s->mb_y == (s->mb_height - 1))        \
             mquant = v->altpq;                                 \
-        if (!mquant || mquant > 31) {                          \
+        if (!mquant || mquant > 31 || mquant < -31) {                          \
             av_log(v->s.avctx, AV_LOG_ERROR,                   \
                    "Overriding invalid mquant %d\n", mquant);  \
             mquant = 1;                                        \