SAX XMLReader error: The entity "ouml" was referenced, but not declared.

I have lots of German umlauts in my html file like this:

`

Zwölf Boxkämpfer

Trying to convert that file to PDF like mentioned here: https://github.com/danfickle/openhtmltopdf/wiki/Integration-Guide#minimal-usage leads to an error: `The entity "ouml" was referenced, but not declared.`

Using the doctype clause mentioned here: https://github.com/danfickle/openhtmltopdf/wiki/Integration-Guide#character-entities gives me this error: `DOCTYPE clause cannot have a non-null PUBLIC ID and a null SYSTEM ID`

Any ideas?

Thanks