diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index b768d38cae..5a76918e29 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -121,7 +121,7 @@ static void png_put_interlaced_row(uint8_t *dst, int width,
 static void add_bytes_l2_c(uint8_t *dst, uint8_t *src1, uint8_t *src2, int w)
 {
     long i;
-    for(i=0; i<=w-sizeof(long); i+=sizeof(long)){
+    for(i=0; i<=w-(int)sizeof(long); i+=sizeof(long)){
         long a = *(long*)(src1+i);
         long b = *(long*)(src2+i);
         *(long*)(dst+i) = ((a&pb_7f) + (b&pb_7f)) ^ ((a^b)&pb_80);