Unhandled Java Exception in create_output()

Here's the full error, first of all:

Unhandled Java Exception in create_output():
java.lang.RuntimeException: Root element is not Bookmark.
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.XfdfReader.startElement(XfdfReader.java:185)
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.SimpleXMLParser.parse(SimpleXMLParser.java:432)
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.SimpleXMLParser.parse(SimpleXMLParser.java:199)
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.SimpleXMLParser.parse(SimpleXMLParser.java:156)
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.XfdfReader.<init>(XfdfReader.java:103)
	at com.gitlab.pdftk_java.filter.create_output_filter(filter.java:140)
	at com.gitlab.pdftk_java.TK_Session.create_output(TK_Session.java:1457)
	at com.gitlab.pdftk_java.pdftk.main_noexit(pdftk.java:190)
	at com.gitlab.pdftk_java.pdftk.main(pdftk.java:161)
There was a problem with pdftk-java. Please report it at
https://gitlab.com/pdftk-java/pdftk/issues
including the message above, the version of pdftk-java (3.3.2), and if possible steps to reproduce the error

As the message says, the version is 3.3.2.

How to reproduce

This occurrs when attempting to fill a valid PDF Acroform (that in my specific case only has a single input field) with a malformed FDF file (it is, in this case, missing the "%FDF-1.2" at the start). In the previous version, 3.3.1, I instead got this output:

Error: Failed read form data on stdin.
   No output created.

This is what I'd expect to still happen (or maybe a more accurate error message) instead of an unhandled Java exception.

It's not super important that this works, as it is only as part of a unit test suite, and I don't plan to regularly provide malformed FDF files. However, I think a normal error output can still reasonably be expected.