Make sure you have “qtrle” selected.
From: https://en.wikipedia.org/wiki/QuickTime_Animation
QuickTime Animation format (also known as QuickTime RLE ) is a video compression format and codec created by Apple Computer to enable playback of RGB video in real time without expensive hardware.[1][2] It is generally found in the QuickTime container with the FourCC 'rle '.[note 1] It can perform either lossless or lossycompression and is one of the few video codecs that supports an alpha channel.