Custom fields cannot be included in the project release notes

I want to enhance the release notes report, so I tried to add a custom defined field in the velocity template releasenotes-text.vm

Using
$issue.getString("<custom field>")
in releasenotes-text.vm I get an exception.

Aren't the custom fields available via $issue ?
It doesn't seem to be, because they are not listed via
$issue.getAllFields().