opencv and python 3 -- cannot write video frames to disk

Python 3.5.2, anaconda 4.2.0 on Windows 10. OpenCV installed from conda, version 3.1.0.

I'm trying to process a video file by opening it, transforming each frame, and putting the result into a new video file. The output file is created, but the size is about 800 bytes and its empty. The input file has ~4,000 frames and it's about 150 MB.

Here's my code, which follows the guide on the OpenCV documentation pretty closely.

import cv2
import progressbar
# preprocess video
# args.input is a valid file name     
    outname = 'foo.mp4'
    cap = cv2.VideoCapture(args.input)    

    codec = int(cap.get(cv2.CAP_PROP_FOURCC)) 
    framerate = app_config.camera.framerate #240
    size = (app_config.camera.width, app_config.camera.height) #1080 x 720

    vw = cv2.VideoWriter(filename=outname, fourcc=codec, fps=framerate, frameSize=size, isColor=False)
    curframe = 0
    with progressbar.ProgressBar(min_value=0, max_value=int(cap.get(cv2.CAP_PROP_FRAME_COUNT))) as pb:
        while cap.isOpened():
            ret, frame = cap.read()
            if ret:
                #update the progress bar
                curframe += 1
                pb.update(curframe)
                # convert to greyscale
                grey = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
                # invert colors
                inverted = cv2.bitwise_not(grey)
                vw.write(inverted)   

                #cv2.imshow('right', right)
                #if cv2.waitKey(1) & 0xFF == ord('q'):
                #    break
            else:
                break

        cap.release()
        vw.release()
        cv2.destroyAllWindows()

I receive the following error:

OpenCV: FFMPEG: tag 0x7634706d/'mp4v' is not supported with codec id 13 and format 'mp4 / MP4 (MPEG-4 Part 14)'
OpenCV: FFMPEG: fallback to use tag 0x00000020/' ???'

I receive similar errors (as well as a warning that I have an incorrect environment variable for h.264 library path) if i try to set codec = cv2.VideoWriter_fourcc(*'H264').

Ensure that the dimensions of inverted match the dimensions of the size parameter in the videoWriter definition.

Also use 'M','P','4','V' codec with the .mp4 container.