diff --git a/libavcodec/lagarith.c b/libavcodec/lagarith.c
index 71e7e99e31..710558cbef 100644
--- a/libavcodec/lagarith.c
+++ b/libavcodec/lagarith.c
@@ -443,7 +443,7 @@ static int lag_decode_arith_plane(LagarithContext *l, uint8_t *dst,
             offset += 4;
         }
 
-        init_get_bits(&gb, src + offset, src_size * 8);
+        init_get_bits(&gb, src + offset, (src_size - offset) * 8);
 
         if (lag_read_prob_header(&rac, &gb) < 0)
             return -1;