Stray rubocop:enable directive causes Rubocop to fail

[koppen]

When running rubocop on a file that contains a leftover rubocop:enable all directive, rubocop triggers an error instead of a cop warning.

Steps to reproduce the problem

1. Create a file containing:

puts
# rubocop:enable all

2. Run Rubocop on that file

$ rubocop file.rb

Expected behavior

Inspecting 1 file
.

1 file inspected, no offenses detected

... or alternatively

1 file inspected, 1 offense detected
Unnecessary `rubocop:enable` directive found

... or something to that effect.

Actual behavior

Inspecting 1 file
An error occurred while Lint/UnneededCopEnableDirective cop was inspecting /Users/jakob/file.rb.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Lint/UnneededCopEnableDirective cop was inspecting /Users/jakob/file.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/bbatsov/rubocop/issues

Mention the following information in the issue report:
0.54.0 (using Parser 2.5.0.4, running on ruby 2.5.0 x86_64-darwin15)

With debugging flag

$ rubocop -d file.rb
For /Users/jakob: configuration from ~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/config/default.yml
Inheriting configuration from ~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/config/enabled.yml
Inheriting configuration from ~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/config/disabled.yml
Inspecting 1 file
Scanning ~/file.rb
An error occurred while Lint/UnneededCopEnableDirective cop was inspecting ~/file.rb.
nil can't be coerced into Integer
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/lint/unneeded_cop_enable_directive.rb:47:in `+'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/lint/unneeded_cop_enable_directive.rb:47:in `range_of_offense'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/lint/unneeded_cop_enable_directive.rb:32:in `block in investigate'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/lint/unneeded_cop_enable_directive.rb:29:in `each'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/lint/unneeded_cop_enable_directive.rb:29:in `investigate'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:100:in `block (2 levels) in invoke_custom_processing'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:109:in `with_cop_error_handling'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:99:in `block in invoke_custom_processing'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:96:in `each'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:96:in `invoke_custom_processing'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/commissioner.rb:58:in `investigate'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/team.rb:114:in `investigate'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/team.rb:102:in `offenses'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cop/team.rb:44:in `inspect_file'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:259:in `inspect_file'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:206:in `block in do_inspection_loop'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:238:in `block in iterate_until_no_changes'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:231:in `loop'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:231:in `iterate_until_no_changes'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:202:in `do_inspection_loop'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:111:in `block in file_offenses'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:121:in `file_offense_cache'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:109:in `file_offenses'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:100:in `process_file'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:78:in `block in each_inspected_file'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:75:in `each'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:75:in `reduce'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:75:in `each_inspected_file'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:67:in `inspect_files'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/runner.rb:39:in `run'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cli.rb:156:in `execute_runner'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cli.rb:84:in `execute_runners'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/lib/rubocop/cli.rb:41:in `run'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/bin/rubocop:13:in `block in <top (required)>'
~/.rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'
~/.rvm/gems/ruby-2.5.0/gems/rubocop-0.54.0/bin/rubocop:12:in `<top (required)>'
~/.rvm/gems/ruby-2.5.0/bin/rubocop:23:in `load'
~/.rvm/gems/ruby-2.5.0/bin/rubocop:23:in `<main>'
~/.rvm/gems/ruby-2.5.0/bin/ruby_executable_hooks:15:in `eval'
~/.rvm/gems/ruby-2.5.0/bin/ruby_executable_hooks:15:in `<main>'
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Lint/UnneededCopEnableDirective cop was inspecting ~/file.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/bbatsov/rubocop/issues

Mention the following information in the issue report:
0.54.0 (using Parser 2.5.0.4, running on ruby 2.5.0 x86_64-darwin15)
Finished in 0.2657470899866894 seconds

RuboCop version

$ rubocop -V
0.54.0 (using Parser 2.5.0.4, running on ruby 2.5.0 x86_64-darwin15)