Sass parse error when assigning a variable

[alkos333]

This problem occurs in sass 3.1.12 on Ruby 1.9.1. The following error is displayed when I try to define a variable in sass interactive shell like so: $padding: 2em;

NoMethodError: undefined method `size' for nil:NilClass
Use --trace for backtrace.

It looks like the size method used inside parse_input method on line 38 of repl.rb isn't defined.
Here's the output for sass -i --trace: https://gist.github.com/1503497