diff --git a/libavformat/avidec.c b/libavformat/avidec.c
index 78a6deae7d..e8c273062a 100644
--- a/libavformat/avidec.c
+++ b/libavformat/avidec.c
@@ -126,7 +126,7 @@ static inline int get_duration(AVIStream *ast, int len)
 {
     if (ast->sample_size)
         return len;
-    else if (ast->dshow_block_align)
+    else if (ast->dshow_block_align > 1)
         return (len + ast->dshow_block_align - 1) / ast->dshow_block_align;
     else
         return 1;