Biber exits with code 25 but no error message

Question

Biber 1.8 has mysteriously stopped working for me:

% biber --trace mytest || echo "Failed with code $?" 
INFO - This is Biber 1.8
INFO - Logfile is 'mytest.blg'
INFO - Reading 'mytest.bcf'
INFO - Using all citekeys in bib section 0
INFO - Processing section 0
INFO - Looking for bibtex format file 'mytest.bib' for section 0
INFO - Decoding LaTeX character macros into UTF-8
INFO - Found BibTeX data source 'mytest.bib'
Failed with code 25

As you can see, there is no error message.

I keep my document in a git repository, and so I was able to test it with some older versions, which fail in the same way, although they used to work (with the same version of Biber). I'm using TeXLive 2014, installed with MacPorts, which reports the version as "1.8_0+perl5_16".

I managed to narrow down the .bib file to a single line that causes the problem, and include these files below. By inserting print statements into the .pm files, I discovered that the failure happens while mappings are being applied to the data read from the .bib file. Specifically, it happens when biber_decode_utf8 in Biber/Utils.pm calls decode_utf8 with a 6 element string consisting of the letters "Citta" followed by the grave accent combining mark (U+0300).

Another question on Biber problems suggests deleting the cache, but biber --cache tells me that I'm not using the binary version of Biber, so there's no cache. This question about a SIGSEGV on Linux suggests rebuilding the dependencies of the package. I did sudo port build depof:biblatex-biber, which had no effect. I've also rebooted, and uninstalled and reinstalled biblatex-biber twice (with MacPorts). Any suggestions?

Here follow the files with which I can reproduce the problem:

mytest.bib:

%% This BibTeX bibliography file was created using BibDesk.
%% Saved with string encoding Unicode (UTF-8) 

@incollection{archivi,
    Address = {Citt{\`a}}}

mytest.bcf is too big to include here. I've posted it as a Gist.