diff --git a/sys/src/avfilter/filter.rs b/sys/src/avfilter/filter.rs
index f44bd94..673ec03 100644
--- a/sys/src/avfilter/filter.rs
+++ b/sys/src/avfilter/filter.rs
@@ -68,11 +68,11 @@ pub struct AVFilterPad {
 	pub kind: AVMediaType,
 	pub min_perms: c_int,
 	pub rej_perms: c_int,
-	pub start_frame: extern fn(*mut AVFilterLink, *mut AVFilterBufferRef) -> c_int,
-	pub get_video_buffer: extern fn(*mut AVFilterLink, c_int, c_int) -> *mut AVFrame,
-	pub get_audio_buffer: extern fn(*mut AVFilterLink, c_int) -> *mut AVFrame,
-	pub end_frame: extern fn(*mut AVFilterLink) -> c_int,
-	pub draw_slice: extern fn(*mut AVFilterLink, c_int, c_int, c_int) -> c_int,
+	pub start_frame: Option<extern fn(*mut AVFilterLink, *mut AVFilterBufferRef) -> c_int>,
+	pub get_video_buffer: Option<extern fn(*mut AVFilterLink, c_int, c_int) -> *mut AVFrame>,
+	pub get_audio_buffer: Option<extern fn(*mut AVFilterLink, c_int) -> *mut AVFrame>,
+	pub end_frame: Option<extern fn(*mut AVFilterLink) -> c_int>,
+	pub draw_slice: Option<extern fn(*mut AVFilterLink, c_int, c_int, c_int) -> c_int>,
 	pub filter_frame: extern fn(*mut AVFilterLink, *mut AVFrame) -> c_int,
 	pub poll_frame: extern fn(*mut AVFilterLink) -> c_int,
 	pub request_frame: extern fn(*mut AVFilterLink) -> c_int,