Comment 8 for bug 1120322

So, this is the loop http://bazaar.launchpad.net/~aptdaemon-developers/aptdaemon/main/view/head:/aptdaemon/gtk3widgets.py#L1114

        for line in difflib.unified_diff(from_lines, to_lines, lineterm=""):
            if line.startswith("@@"):
                match = re.match(self.REGEX_RANGE, line)
                if not match:
                    continue
                line_number = int(match.group("from_start"))
                if line_number > 1:
                    insert_tagged_text(iter, self.ELLIPSIS, "default")
            elif line.startswith("---") or line.startswith("+++"):
                continue

Note the 'if not match: continue' after the REGEX_RANGE lookup - and REGEX_RANGE is:
    REGEX_RANGE = "^@@ \-(?P<from_start>[0-9]+),(?P<from_context>[0-9]+) " \
                  "\+(?P<to_start>[0-9]+),(?P<to_context>[0-9]+) @@"

Now difflib.py has this for its output:
        first, last = group[0], group[-1]
        file1_range = _format_range_unified(first[1], last[2])
        file2_range = _format_range_unified(first[3], last[4])
        yield '@@ -{} +{} @@{}'.format(file1_range, file2_range, lineterm)

And if you look at _format_range_unified it has this:
    # Per the diff spec at http://www.unix.org/single_unix_specification/
    beginning = start + 1 # lines start numbering with one
    length = stop - start
    if length == 1:
        return '{}'.format(beginning)
    if not length:
        beginning -= 1 # empty ranges begin at line just before the range
    return '{},{}'.format(beginning, length)

So if the diff range is precisely 1 line long, the output will be
@@ -12 +12,13 @@
But the regex aptdaemon has expects x,y never just x.

So - bad regex, fix that and you should be good. May need to tweak the assignment code of course.