diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
index 2ea55ef441..945ed121dd 100644
--- a/libavfilter/vf_scale.c
+++ b/libavfilter/vf_scale.c
@@ -81,6 +81,7 @@ typedef struct {
      * New dimensions. Special values are:
      *   0 = original width/height
      *  -1 = keep original aspect
+     *  -N = try to keep aspect but make sure it is divisible by N
      */
     int w, h;
     char *size_str;