dotPeek is not allowed to pen dll files

I'm using dotPeek 2019.3.4, and everytime I try to open a .dll using the right click menu in Explorer I get this error message:

You are attempting to open a file of type 'System File' (.dll) and then I must select a new program, e.g. ILSpy which opens the .dll!

If I start dotPeek then I can open .dll files

Why is this happening and how do I fix it?

// Anders