diff --git a/libavfilter/avfilter.h b/libavfilter/avfilter.h
index d296536151..296f2d3c9a 100644
--- a/libavfilter/avfilter.h
+++ b/libavfilter/avfilter.h
@@ -1245,6 +1245,8 @@ typedef struct AVFilterGraph {
 
 /**
  * Allocate a filter graph.
+ *
+ * @return the allocated filter graph on success or NULL.
  */
 AVFilterGraph *avfilter_graph_alloc(void);