Validation does not work for nested nodes

We have created a type alinea that is used in poc:document and therefore in poc:nieuwsbericht. When saving a nieuwbericht, the title of the alinea is not validated. The title of the nieuwsbericht is validated. Our cnd file looks like this:

[poc:alinea]

• poc:titel (string) mandatory
• poc:tekst (string)
  + * (hippo:facetselect)

[poc:imagelink] > hippo:facetselect

• poc:alt (string)

[poc:document] > hippostd:publishable, hippostd:publishableSummary, hippo:document

• poc:titel (string) mandatory
• poc:introductie (string)
• poc:ankers (boolean)
  + poc:alinea (poc:alinea) multiple
  + poc:afbeelding (poc:imagelink)
• poc:documentdatum (date) mandatory

[poc:nieuwsbericht] > poc:document