diff --git a/libswscale/utils.c b/libswscale/utils.c
index e1ad685972..e6eab0c4c1 100644
--- a/libswscale/utils.c
+++ b/libswscale/utils.c
@@ -1891,7 +1891,7 @@ static av_cold int sws_init_single_context(SwsContext *c, SwsFilter *srcFilter,
     }
 
     for (i = 0; i < 4; i++)
-        if (!FF_ALLOCZ_TYPED_ARRAY(c->dither_error[i], c->dstW + 2))
+        if (!FF_ALLOCZ_TYPED_ARRAY(c->dither_error[i], c->dstW + 3))
             goto nomem;
 
     c->needAlpha = (CONFIG_SWSCALE_ALPHA && isALPHA(c->srcFormat) && isALPHA(c->dstFormat)) ? 1 : 0;