Video Goes Black when I Try to Slow it Down

Hi

I created a sequence that I rendered as a QT video. Unfortunately, the movements were keyframed too fast.

Following an online help post; In Video Editing, I took my image sequence, Add -> Effect Strip -> Speed Control, “Stretch to input strip length” (I’ve also tried “Use as speed”), put .5 into Multiply Speed, and hit Render Animation. On one try, I left my frame number as is (650), another time I multiplied by 2 (1300).

if my video is at 650 frames, it ends halfway through the animation.
If my video is at 1300 frames, it ends halfway through the animation AND the last half is a black screen.

I’m baffled. What am I doing wrong? Do I have to start the key framing all over? (Because I’d much rather not; it took half a month to render the frames.)

Thanks,
AerialBot