diff --git a/libavcodec/mdec.c b/libavcodec/mdec.c
index e91d5b7858..ed2476213d 100644
--- a/libavcodec/mdec.c
+++ b/libavcodec/mdec.c
@@ -158,7 +158,7 @@ static inline void idct_put(MDECContext *a, int mb_x, int mb_y){
 
 static int decode_frame(AVCodecContext *avctx,
                         void *data, int *data_size,
-                        uint8_t *buf, int buf_size)
+                        const uint8_t *buf, int buf_size)
 {
     MDECContext * const a = avctx->priv_data;
     AVFrame *picture = data;
diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
index 5b006c3c0f..87f673d121 100644
--- a/libavcodec/mpeg12.c
+++ b/libavcodec/mpeg12.c
@@ -2220,7 +2220,7 @@ int ff_mpeg1_find_frame_end(ParseContext *pc, const uint8_t *buf, int buf_size)
 /* handle buffering and image synchronisation */
 static int mpeg_decode_frame(AVCodecContext *avctx,
                              void *data, int *data_size,
-                             uint8_t *buf, int buf_size)
+                             const uint8_t *buf, int buf_size)
 {
     Mpeg1Context *s = avctx->priv_data;
     const uint8_t *buf_end;