Inkscape unable to print with cups and brother laser printer

[Eric Saint-Etienne]

On ubuntu 16.04, printing from inkscape on a Brother printer fails with the following message in /var/log/cups/error_log:

[...]
D [13/Dec/2017:21:50:41 +0000] [Job 87] Can\'t use an undefined value as a subroutine reference at /usr/lib/cups/filter/brother_lpdwrapper_MFCL2700DW line 565.
[...]
D [13/Dec/2017:21:50:41 +0000] [Job 87] printer-state-message="Filter failed"
[...]

The error happens in the perl cups filter /usr/lib/cups/filter/brother_lpdwrapper_MFCL2700DW.
This perl script seem to fail because it did not get a valid value for the unit:

549 if ( $lpr_options_ref->{'-pt'} eq "CUSTOM0" ){
550
551 foreach (keys %units){
552
553 if ( $ppdcust =~ /[0-9]$_$/) {
554 logprint( 2 , "HIT $_\n");
555 $unit = $_;
556 $unitfunc = $units{$_};
557 }
558 }
559 $ssize = $ppdcust;
560 $ssize =~ s/^Custom\.//g;
561 $ssize =~ s/$unit$//g;
562
563
564 my @psiz = split (/x/,$ssize);
565 ($x , $y) = &$unitfunc ( $psiz[0] ,$psiz[1]);
566
567 return ($x , $y);
568 }

At runtime I do have $lpr_options_ref->{'-pt'} equal to "CUSTOM0", so the script enters the code block, but when looping through the keys of %units, perl can't find a matching key. That's because $unit is empty (""), whereas a human readable unit is expected. See definition of the %unit hash:
 56 %units = ( pt => \&pt2dot,
 57 mm => \&mm2dot,
 58 inch => \&inch2dot,
 59 in => \&inch2dot,
 60 cm => \&cm2dot,
 61 m => \&m2dot,
 62 ft => \&ft2dot
 63 );

I'm not sure if it's because inkscape doesn't provide a valid unit when using CUSTOM0 or if it's a bug of the brother filter (which should assume some default unit).

Printing the same SVG file from Chrome works fine.

I'm using the following package versions:
- inkscape version 0.91-7ubuntu2
- mfcl2700dwcupswrapper:i386 version 3.2.0-1 (the latest)
- mfcl2700dwlpr:i386 version 3.2.0-1 (the latest)
- cups-common version 2.1.3-4ubuntu0.3
- cups-core-drivers version 2.1.3-4ubuntu0.3
- cups-server-common version 2.1.3-4ubuntu0.3
- cups-filters version 1.8.3-2ubuntu3.1
- cups-filters-core-drivers version 1.8.3-2ubuntu3.1
- libcupsfilters1:amd64 version 1.8.3-2ubuntu3.1
- libcups2:amd64 version 2.1.3-4ubuntu0.3
- libcups2:i386 version 2.1.3-4ubuntu0.3

Thank you.

-eric