Issue #2365 has been updated by Keiju Ishitsuka.

Status changed from Assigned to Rejected

This behavior is the same as an integer: 

% ruby-dev -r matrix -e "p 2 - nil"
-e:1:in `-': nil can't be coerced into Fixnum (TypeError)
        from -e:1:in `<main>'

I think that these behavior is Ruby's standard behavior.
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http://redmine.ruby-lang.org/issues/show/2365

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