Nested Structure Fields?

The reference page for the structure fields states, that it is possible to define a structure field inside a structure field →

I tried setting up a nested structure field, in order to allow a user to create categories and then create items (consisting of multiple fields, hence the structure field inside the structure field) inside those categories:

  label: Personal Projects
  type: structure
      label: Project Categories
      type: structure
            label: Project Info
            type: text
            label: Project Client
            type: text
            label: Year
            type: date

Unfortunately, I get the following error when trying to enter something into the field:

Where am I making a mistake? Or is it actually not possible to put structure fields inside structure fields?

Your indentation is incorrect, type and fields of the nested field should have the same indent, just like you did it for the upper level structure field.

Of course, simple mistake. Sorry for overlooking it and thanks for pointing it out.