diff --git a/libavcodec/4xm.c b/libavcodec/4xm.c
index e2798587af..aca86181ba 100644
--- a/libavcodec/4xm.c
+++ b/libavcodec/4xm.c
@@ -340,7 +340,7 @@ static int decode_p_frame(FourXContext *f, uint8_t *buf, int length){
     const int stride= f->current_picture.linesize[0]>>1;
     unsigned int bitstream_size, bytestream_size, wordstream_size, extra;
 
-    if(!get32(buf-4)){
+    if(f->version){
         extra=20;
         bitstream_size= get32(buf+8);
         wordstream_size= get32(buf+12);