diff --git a/libavutil/avutil.h b/libavutil/avutil.h
index d71eb1ef4c..fd9bdc88a7 100644
--- a/libavutil/avutil.h
+++ b/libavutil/avutil.h
@@ -261,7 +261,7 @@ static inline void *av_x_if_null(const void *p, const void *x)
  * @return  length of the list, in elements, not counting the terminator
  */
 unsigned av_int_list_length_for_size(unsigned elsize,
-                                     const void *list, uint64_t term);
+                                     const void *list, uint64_t term) av_pure;
 
 /**
  * Compute the length of an integer list.