Adding subs to MP4 results in added black bars

[Cookett]

Hello everyone, I wondered a lot on the web to find a solution to my problem but find nothing.
I use mkvtoolnix to add .srt files to .MP4 and I like a lot the gui.
However the output video file has extra black borders on the side.
The starting video is 16:9 with top and bottom black bars, but during the muxing process it adds bars also on the side.
How should I prevent that?
Thanks for your help

[jagabo]

Set the display aspect ratio for the video stream in mkvtoolnix.