Uncaught TypeError: Cannot read property 'indexOf' of undefined

[Susalko]

[Enter steps to reproduce:]

1. ...
2. ...

Atom: 1.29.0 x64
Electron: 2.0.5
OS: Unknown Windows version
Thrown From: linter-gcc package 0.7.1

Stack Trace

Uncaught TypeError: Cannot read property 'indexOf' of undefined

At C:\Users\User\.atom\packages\linter-gcc\lib\utility.js:29

TypeError: Cannot read property 'indexOf' of undefined
    at Object.grammarType (/packages/linter-gcc/lib/utility.js:29:29)
    at Object.getValidEditor (/packages/linter-gcc/lib/utility.js:38:25)
    at lintOnTheFly (/packages/linter-gcc/lib/main.js:165:24)
    at Function.module.exports.Emitter.simpleDispatch (~/AppData/Local/atom/app-1.29.0/resources/app/node_modules/event-kit/lib/emitter.js:27:20)
    at Emitter.module.exports.Emitter.emit (~/AppData/Local/atom/app-1.29.0/resources/app/node_modules/event-kit/lib/emitter.js:156:34)
    at TextBuffer.module.exports.TextBuffer.emitDidStopChangingEvent (~/AppData/Local/atom/app-1.29.0/resources/app/node_modules/text-buffer/lib/text-buffer.js:1817:26)
    at later (~/AppData/Local/atom/app-1.29.0/resources/app/node_modules/text-buffer/lib/helpers.js:28:13)

Commands

     -2:51.9.0 tree-view:add-folder (ol.tree-view-root.full-menu.list-tree.has-collapsable-children.focusable-panel)
     -2:50.2.0 core:confirm (input.hidden-input)
  2x -2:35.7.0 intentions:highlight (input.hidden-input)
     -2:35.1.0 command-palette:toggle (input.hidden-input)
 11x -2:29.1.0 core:move-down (input.hidden-input)
 11x -2:25.1.0 core:backspace (input.hidden-input)
 26x -2:12.5.0 core:move-down (input.hidden-input)
     -2:02.4.0 core:move-up (input.hidden-input)
     -2:01.9.0 core:confirm (input.hidden-input)
     -2:01.9.0 git-plus:new-branch (input.hidden-input)
     -1:52.4.0 core:confirm (input.hidden-input)
     -1:35.3.0 command-palette:toggle (atom-pane.pane)
  2x -1:27.4.0 core:move-down (input.hidden-input)
     -1:19 core:confirm (input.hidden-input)
     -1:19 open-on-github:branch-compare (atom-pane.pane)
  2x -0:15.5.0 core:backspace (input.hidden-input)

Non-Core Packages

advanced-open-file 0.16.8 
atom-beautify 0.33.0 
autoclose-html 0.23.0 
busy-signal 1.4.3 
file-icons 2.1.24 
git-log 0.4.1 
git-plus 8.0.1 
intentions 1.1.5 
language-ini 1.20.0 
linter 2.2.0 
linter-gcc 0.7.1 
linter-ui-default 1.7.1 
sf-tools 1.0.6 
tool-bar 1.1.7