libsass accept $map+1,ruby sass not

[iddekingej]

Code:

$map:(a:b,c:d);
$qq:$map+1;
@debug($qq)

output sassc:
DEBUG: (a: b, c: d)1

output ruby sass:
Error: (a: b, c: d) isn't a valid CSS value.

sassc version:
sassc: 3.3.6-11-g077a
libsass: 3.3.6-92-g014a61
sass2scss: 1.0.6
sass: 3.4

ruby sass version
Sass 3.4.22.7716e67