doesnt work correctly if length is null

currently if length is null or blank,
!isNan(options.length) return true, 
and length = parseInt(options.length, 10); makes length NaN, which doesnt matches with subsequent length != null check in various places

Coz of this we are facing following issue
https://github.com/visionmedia/express/issues/1816#issuecomment-35760090