NullPointerException when using fx:include

FXMLLoader fails to load a sub layout using fx:include.

Here is the parent.fxml:

<AnchorPane xmlns:fx="http://javafx.com/fxml">
  <children>
    <fx:include source="/include.fxml"/>
  </children>
</AnchorPane>

Here is the include.fxml (empty pane):

<AnchorPane xmlns:fx="http://javafx.com/fxml">
</AnchorPane>

At runtime I get the following stacktrace:

Caused by: java.lang.NullPointerException
at javafx.fxml.FXMLLoader.equals(FXMLLoader.java:1855)
at javafx.fxml.FXMLLoader.isCyclic(FXMLLoader.java:1867)
at javafx.fxml.FXMLLoader.access$2200(FXMLLoader.java:66)
at javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:935)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:565)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2348)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2164)