diff --git a/libavcodec/dirac_parser.c b/libavcodec/dirac_parser.c
index 83c35a2010..12f1a60145 100644
--- a/libavcodec/dirac_parser.c
+++ b/libavcodec/dirac_parser.c
@@ -100,10 +100,12 @@ typedef struct DiracParseUnit {
 static int unpack_parse_unit(DiracParseUnit *pu, DiracParseContext *pc,
                              int offset)
 {
-    uint8_t *start = pc->buffer + offset;
-    uint8_t *end   = pc->buffer + pc->index;
-    if (start < pc->buffer || (start + 13 > end))
+    int8_t *start;
+
+    if (offset < 0 || pc->index - 13 < offset)
         return 0;
+
+    start = pc->buffer + offset;
     pu->pu_type = start[4];
 
     pu->next_pu_offset = AV_RB32(start + 5);