diff --git a/libavfilter/avfilter.h b/libavfilter/avfilter.h
index 1b42086611..5c41630909 100644
--- a/libavfilter/avfilter.h
+++ b/libavfilter/avfilter.h
@@ -1004,6 +1004,8 @@ typedef struct AVFilterGraph {
 
 /**
  * Allocate a filter graph.
+ *
+ * @return the allocated filter graph on success or NULL.
  */
 AVFilterGraph *avfilter_graph_alloc(void);