diff --git a/include/SDL_audio.h b/include/SDL_audio.h
index e09a37554f..936b78aa64 100644
--- a/include/SDL_audio.h
+++ b/include/SDL_audio.h
@@ -154,6 +154,7 @@ typedef Uint16 SDL_AudioFormat;
  * \param userdata An application-specific parameter saved in the
  *                 SDL_AudioSpec structure
  * \param stream A pointer to the audio data buffer.
+ * \param len Length of **stream** in bytes.
  */
 typedef void (SDLCALL * SDL_AudioCallback) (void *userdata, Uint8 * stream,
                                             int len);