keying effect disappears during video clip fade out

I'm pretty new to FCP and I decided to try a green screen effect in my latest video. My top image was me at the piano (the clip I applied the keyer effect to). The clip below was snowfall. (I wanted it to look like I was playing my piano in front of a picture window during a snowfall)

Everything looked the way I wanted. But when I put a cross dissolve at the end of the piano clip to make it a 7 second fad out the "snowfall" disappeared... and in its place was mostly a black matte with a little white (showing where the lighting of my green screen was not even)

I worked with Sony Vegas for years and never experienced this problem. Does anyone have a solution to this?

This is a link to the video. Feel free to fast forward to the last 10 seconds or so to see what I'm describing.



Any help is appreciated.

cheers,
Jeff

Put a clip of black over everything and dissolve into that.

That "disappearing" of the key is typical when using transitions that are "one sided" (in your case, the incoming clip into the transition).  Filling the outgoing end with a gap does not work, however a slug will.

A slug is just "blank" video (or it can be a transparent image.) You can download a free slug here:

fcpxtemplates.com/video-slug/

The benefit of the slug over a transparent image is that it will adapt to any sized project, whereas you generally need a specific aspect ratio dimension for a transparent image to fit whatever the project size you're using.

You could also just use the Video animation editor tool (select your clip and type Control-V) and adjust the outgoing Opacity [this is much easier than keyframing the Opacity in the Effects Inspector.] If you're unfamiliar with this tool, when you open up the workspace, at the very bottom is Opacity. On the right side of its titlebar is a small icon with a downward pointing triangle. Click that to open the Opacity editor. On each edge of the editor will be small circles that you can click and drag inward to create a Fade animation.

HTH