diff --git a/musicalgestures/_motionvideo.py b/musicalgestures/_motionvideo.py
index 1dd422b7..98da0034 100644
--- a/musicalgestures/_motionvideo.py
+++ b/musicalgestures/_motionvideo.py
@@ -248,6 +248,7 @@ def mg_motion(
 
             if save_video:
                 if video_out is None:
+                    assert target_name_video is not None  # resolved under `if save_video` above
                     cmd =['ffmpeg', '-y', '-s', '{}x{}'.format(motion_frame.shape[1], motion_frame.shape[0]), 
                         '-r', str(self.fps), '-f', 'rawvideo', '-pix_fmt', 'bgr24', '-vcodec', 'rawvideo', 
                         '-i', '-', '-vcodec', 'libx264', '-pix_fmt', 'yuv420p', target_name_video]
@@ -265,6 +266,8 @@ def mg_motion(
 
         # Terminate the processes
         if save_video:
+            # the writer opens on the first frame, so this only holds for a video that had frames
+            assert video_out is not None, "no frames were written: the video decoded to nothing"
             video_out.stdin.close()
             video_out.wait()
         process.terminate()
diff --git a/musicalgestures/_pose.py b/musicalgestures/_pose.py
index c03f06df..a7c9d8b8 100644
--- a/musicalgestures/_pose.py
+++ b/musicalgestures/_pose.py
@@ -565,6 +565,7 @@ def pose(
 
         if save_video:
             if video_out is None:
+                assert target_name_video is not None  # resolved under `if save_video` above
                 cmd =['ffmpeg', '-y', '-s', '{}x{}'.format(frame.shape[1], frame.shape[0]), 
                     '-r', str(self.fps), '-f', 'rawvideo', '-pix_fmt', 'bgr24', '-vcodec', 'rawvideo', 
                     '-i', '-', '-vcodec', 'libx264', '-pix_fmt', 'yuv420p', target_name_video]
@@ -579,6 +580,8 @@ def pose(
 
     # Terminate the processes
     if save_video:
+        # the writer opens on the first frame, so this only holds for a video that had frames
+        assert video_out is not None, "no frames were written: the video decoded to nothing"
         video_out.stdin.close()
         video_out.wait()
         # Check if the original video fil has audio
@@ -1205,7 +1208,7 @@ def _pose_mediapipe(
         # Collect data row: time + normalised (x, y) for every landmark.
         # Always collected so the images and the keypoint cache are available.
         time_ms = frame2ms(ii, self.fps)
-        row = [time_ms]
+        row: list[float] = [time_ms]
         for i in range(len(MEDIAPIPE_LANDMARK_NAMES)):
             x, y, vis = keypoints[i]
             if vis >= threshold:
@@ -1260,6 +1263,8 @@ def _pose_mediapipe(
         estimator.close()
 
     if save_video:
+        # the writer opens on the first frame, so this only holds for a video that had frames
+        assert video_out is not None, "no frames were written: the video decoded to nothing"
         video_out.stdin.close()
         video_out.wait()
         if self.has_audio:
diff --git a/musicalgestures/_utils.py b/musicalgestures/_utils.py
index 0b6600a9..85e6885e 100644
--- a/musicalgestures/_utils.py
+++ b/musicalgestures/_utils.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import warnings
-from typing import Union, Tuple, TYPE_CHECKING, cast
+from typing import Any, Union, Tuple, TYPE_CHECKING, cast
 
 if TYPE_CHECKING:
     # numpy is imported lazily inside functions (import-speed optimization); make the name
@@ -1845,7 +1845,7 @@ def __init__(self, message):
         self.message = message
 
 
-def ffmpeg_cmd(command: list, total_time: float, pb_prefix: str = 'Progress', print_cmd: bool = False, stream: bool = True, pipe: str | None = None):
+def ffmpeg_cmd(command: list, total_time: float, pb_prefix: str = 'Progress', print_cmd: bool = False, stream: bool = True, pipe: str | None = None) -> Any:
     """
     Run an ffmpeg command in a subprocess and show progress using an MgProgressbar.
 
@@ -1857,6 +1857,16 @@ def ffmpeg_cmd(command: list, total_time: float, pb_prefix: str = 'Progress', pr
         stream (bool, optional): Whether to have a continuous output stream or just (the last) one. Defaults to True (continuous stream).
         pipe (str, optional): Whether to pipe video frames from FFmpeg to numpy array. Possible to read the video frame by frame with pipe='read', to load video in memory with pipe='load', or to write the frames of a numpy array to a video file with pipe='write'. Defaults to None.
 
+    Returns:
+        Any: What comes back depends on `pipe`, and the caller knows which it
+            asked for. `'read'` and `'write'` give a `subprocess.Popen` to read
+            frames from or write frames to, `'load'` gives the
+            `subprocess.CompletedProcess` of a finished run, and the default of
+            None runs the command to completion behind a progress bar and
+            returns nothing. Annotating that precisely would need overloads on
+            the value of a string argument, which would describe the signature
+            more exactly than it deserves.
+
     Raises:
         KeyboardInterrupt: If the user stops the process.
         FFmpegError: If the ffmpeg process was unsuccessful.