My document (LuaLaTeX):

```
\documentclass[a4paper,oneside,12pt]{scrreprt}
\usepackage{fontspec}
\usepackage[backend=biber,firstinits=true,sorting=nyt,maxbibnames=3]{biblatex} 
\usepackage[ngerman]{babel}
\addbibresource{lit.bib}
\begin{document}
\cite{schmidt14}
\printbibliography
\end{document}

```

The file "lit.bib":

```
@phdthesis{schmidt14,
  author = {Schmidt, Johannes},
  title = {Testtest},
  school = {Test Univ.},
  year = {2014},
}

```

But there is no comma between "Diss." (German abbreviation for a PhD thesis) and the school:

![enter image description here](https://i.stack.imgur.com/bzHLs.png)

What can I do? How can I modify the default style? Other styles put a comma there.

(btw, the 
 `\usepackage[ngerman]{babel}`
doesn't matter, same problem with English. I included it nonetheless for the minimal example. The document is written in German.)

Thanks for any help!