Hi Bastien et. al,
I figured out how to fix the bug i reported awhile ago. The problem was
that when making the keyword field for entries without keywords defined in
the `:PROPERTIES:' drawer,  org-bibtex uses `bibtex-make-field' from
`bibtex.el'. This function sends up an error if there are no fields defined
for the type of bib entry you are calling it on. In order to make it work
on non-standard bib types you would have to define the fields associated
with these types in `bibtex-entry-alist'.
One way to deal with this would be to mention this in the appropriate
docstrings. However this would leave org-bibtex somewhat unstable and
dependent on variables outside of `org'.  I think a better solution is to
make the keyword field without calling `bibtex-make-field'. This can be
done with a couple simple regexp functions.

Here is the patch that I came up with. Let me know what you think.
All best,
Leonard