diff --git a/libavcodec/webp.c b/libavcodec/webp.c
index 8281cc45b4..50a8da1000 100644
--- a/libavcodec/webp.c
+++ b/libavcodec/webp.c
@@ -694,7 +694,7 @@ static int decode_entropy_coded_image(WebPContext *s, enum ImageRole role,
                 length = offset + get_bits(&s->gb, extra_bits) + 1;
             }
             prefix_code = huff_reader_get_symbol(&hg[HUFF_IDX_DIST], &s->gb);
-            if (prefix_code > 39) {
+            if (prefix_code > 39U) {
                 av_log(s->avctx, AV_LOG_ERROR,
                        "distance prefix code too large: %d\n", prefix_code);
                 return AVERROR_INVALIDDATA;