Get exception when try to clean string as '@'

[doherty88]

Hello,
You can use following code to reproduce this exception. it is happend in version 3.0.4. And I also tried with 3.0.3, there was no excepiton.
var Filter = require('bad-words'),
filter = new Filter();

console.log(filter.clean("@"));

Thanks
Deyang

[vdfdev]

Same for:
/
!
etc

[johnnyapol]

Appears to be a duplicate of #93