Error: 'weak' used out of scope when running grunt

[francdore]

$ grunt
Running "jshint:files" (jshint) task

source/jarallax.js
72 | if (this.allowWeakProgress || !weak) {
^ 'weak' used out of scope.

1 error in 13 files
Warning: Task "jshint:files" failed. Use --force to continue.

Aborted due to warnings.