parseSequence not working when type is Number

[marchah]

TypeError: this[options.field].substring is not a function
      at model.parseSequence (increment.js:124:40)
      at apply (node_modules/lodash/lodash.js:409:27)
      at model.wrapper [as parseSequence] (node_modules/lodash/lodash.js:4837:16)
      at doc.save.then (test/increment.spec.js:553:24)
      at newTickHandler (node_modules/mongoose/node_modules/mpromise/lib/promise.js:234:18)
      at _combinedTickCallback (internal/process/next_tick.js:73:7)
      at process._tickCallback (internal/process/next_tick.js:104:9)