diff --git a/libavfilter/vf_gradfun.c b/libavfilter/vf_gradfun.c
index a71a68ecc1..e8d9cae828 100644
--- a/libavfilter/vf_gradfun.c
+++ b/libavfilter/vf_gradfun.c
@@ -92,7 +92,7 @@ static void filter(GradFunContext *ctx, uint8_t *dst, const uint8_t *src, int wi
     for (y = 0; y < r; y++)
         ctx->blur_line(dc, buf + y * bstride, buf + (y - 1) * bstride, src + 2 * y * src_linesize, src_linesize, width / 2);
     for (;;) {
-        if (y < height - r) {
+        if (y + 1 < height - r) {
             int mod = ((y + r) / 2) % r;
             uint16_t *buf0 = buf + mod * bstride;
             uint16_t *buf1 = buf + (mod ? mod - 1 : r - 1) * bstride;