Creating a new document causes NPE

While creating a new document with the document node definition

[project:answer]

• project:text (string)
• project:votes (long)

// a poll is not publishable
[project:poll] > hippo:document

• project:title (string)
• project:text (string)
  + project:answer (project:answer) multiple

I get a NPE on creation of the document with the attached stacktrace.