diff --git a/libavfilter/avfilter.c b/libavfilter/avfilter.c
index 7dc8b482be..c5c30448dc 100644
--- a/libavfilter/avfilter.c
+++ b/libavfilter/avfilter.c
@@ -275,10 +275,9 @@ int avfilter_config_links(AVFilterContext *filter)
                     link->sample_aspect_ratio = inlink ?
                         inlink->sample_aspect_ratio : (AVRational){1,1};
 
-                if (inlink && !link->frame_rate.num && !link->frame_rate.den)
-                    link->frame_rate = inlink->frame_rate;
-
                 if (inlink) {
+                    if (!link->frame_rate.num && !link->frame_rate.den)
+                        link->frame_rate = inlink->frame_rate;
                     if (!link->w)
                         link->w = inlink->w;
                     if (!link->h)
diff --git a/libavfilter/avfilter.h b/libavfilter/avfilter.h
index 5d4cd6ce12..7aac3cf934 100644
--- a/libavfilter/avfilter.h
+++ b/libavfilter/avfilter.h
@@ -445,11 +445,12 @@ struct AVFilterLink {
     int age_index;
 
     /**
-     * Frame rate of the stream on the link, or 1/0 if unknown;
-     * if left to 0/0, will be automatically be copied from the first input
+     * Frame rate of the stream on the link, or 1/0 if unknown or variable;
+     * if left to 0/0, will be automatically copied from the first input
      * of the source filter if it exists.
      *
      * Sources should set it to the best estimation of the real frame rate.
+     * If the source frame rate is unknown or variable, set this to 1/0.
      * Filters should update it if necessary depending on their function.
      * Sinks can use it to set a default output frame rate.
      * It is similar to the r_frame_rate field in AVStream.