AttributeError while Batch generate

[Manufraktur]

Hi,
I have two fonts which are interpolatable (I made sure via Prepolator). Then I created a DesignSpace and go to File > Batch > Variable Font, hit Generate. It was working for me in the past, but now I get the error message:
AttributeError: 'NoneType' object has no attribute 'name'
Here a full screenshot:

I have no idea what this message wants to tell me. Is there anything I can do to make it work (meaning to generate a ttf file)?
Thanks in advance,
Petra

[Manufraktur]

Thank you Frederik! You saved me. It's working now :)
Have a great weekend,
Petra

[frederik]

use the designSpace editor and validate your design space :)

you dont have a "default" source: your axis have not a source on the default value of each axes.

hope this makes sense!