Uncaught TypeError: Array.prototype.indexOf called on null or undefined

[weerox]

[Enter steps to reproduce:]

1. ...
2. ...

Atom: 1.18.0 x64
Electron: 1.3.15
OS: Mac OS X 10.12.5
Thrown From: css-spy package 0.6.0

Stack Trace

Uncaught TypeError: Array.prototype.indexOf called on null or undefined

At /Users/WeeRox/.atom/packages/css-spy/lib/css-spy.coffee:68

TypeError: Array.prototype.indexOf called on null or undefined
    at indexOf (native)
    at /packages/css-spy/lib/css-spy.coffee:68:18
    at /packages/css-spy/lib/css-spy.coffee:51:13
    at Function.module.exports.Emitter.simpleDispatch (/Applications/Atom.app/Contents/Resources/app/node_modules/event-kit/lib/emitter.js:25:20)
    at Emitter.module.exports.Emitter.emit (/Applications/Atom.app/Contents/Resources/app/node_modules/event-kit/lib/emitter.js:141:34)
    at /Applications/Atom.app/Contents/Resources/app/node_modules/text-buffer/lib/text-buffer.js:1442:31

Commands

  7x -2:09.2.0 core:undo (input.hidden-input)
     -2:05.1.0 core:backspace (input.hidden-input)
  2x -2:04.9.0 core:save (input.hidden-input)
     -1:52.2.0 tree-view:copy (div.tool-panel.tree-view)
     -1:51.6.0 tree-view:paste (div.tool-panel.tree-view)
     -1:22.7.0 symbols-view:toggle-project-symbols (ul.list-inline.tab-bar.inset-panel)
     -1:16.5.0 tree-view:move (li.file.entry.list-item.status-added.selected)
     -1:15.4.0 core:move-right (input.hidden-input)
  7x -1:14.8.0 core:backspace (input.hidden-input)
     -1:04.7.0 core:confirm (input.hidden-input)
     -0:52.3.0 core:copy (input.hidden-input)
     -0:51.8.0 core:move-right (input.hidden-input)
     -0:51.4.0 editor:newline (input.hidden-input)
     -0:51.3.0 core:paste (input.hidden-input)
  4x -0:14.1.0 core:backspace (input.hidden-input)
  2x -0:04.7.0 core:save (input.hidden-input)

Non-Core Packages

auto-indent 0.5.0 
autoclose-html 0.23.0 
css-spy 0.6.0 
hex-editor 0.1.1 
minimap 4.28.2