diff --git a/libavcodec/4xm.c b/libavcodec/4xm.c
index 158b37a38b..cfe65c178a 100644
--- a/libavcodec/4xm.c
+++ b/libavcodec/4xm.c
@@ -885,6 +885,8 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *picture,
         }
 
         if (i >= CFRAME_BUFFER_COUNT) {
+            if (free_index < 0)
+                return AVERROR_INVALIDDATA;
             i             = free_index;
             f->cfrm[i].id = id;
         }