Hardcoded subs disappearing on Youtube

Hi,

I needed to hardcode custom subs into a video. The trick is however they must have a black textbox in the background; MovieMaker can't do that. I downloaded Aegisubs, which does have the function and made the subs. Next, I used MKVToolNix to merge the subs with the video. It worked: I have a mkv file consisting of the video and the subs. When I'm uploading the video to YouTube, the subs disappear.

This is the 2nd day I'm struggling with this. Help!