[Vue 2.0] text input with number modifier displays NaN value

[sqal]

Vue.js version

2.0.0-alpha.5, 2.0.0-alpha.6 (haven't checked previous releases)

Reproduction Link

https://jsfiddle.net/9mt1fjrg/

Steps to reproduce

Type any letter into input field.

What is Expected?

Shouldn't display NaN value.

[kazupon]

looks like a bug to me.