OpenCV Black and White video empty - c++

Question

I have some code to record a video using openCV. It works fine for recording colour video, but I'd like to record black and white.
When I call cvtColor to black and white I get an empty video. I'd really like to know what I'm doing wrong.
VideoCapture cap(1); // open the default camera
cap.set(CV_CAP_PROP_FPS, fps);
cap.set(CV_CAP_PROP_FRAME_WIDTH, 1280);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 720);
if(!cap.isOpened()) // check if we succeeded
return -1;
VideoWriter writer(filename, CV_FOURCC('M','P','4','2'), fps, Size(1280, 720), true);
int count = 0;
for(;;)
{
count++;
Mat frame;
cap >> frame;
//cvtColor(frame, frame, CV_BGR2GRAY);
writer.write(frame);
}
The above code produces a perfectly fine video, but when cvtColor is uncommented the file is empty.

Answer 1

I was trying to make b/w video with XVID codec and got empty film too (5kB length), until I made FFMPEG libraries available to program (put them together with program or in PATH-ed directory, Windows OS).
OpenCV checks for FFMPEG presence and uses it if available.
And aside remark - use the second Mat for b/w frame - it will save computer time (with single object every capture causes two reallocations/reinitializations)
cvtColor(frame, bwframe, CV_BGR2GRAY);