TypeError: Cannot read property 'length' of undefined

I'm using SilkUI Navigation\Wizard4Step. I have a button to go back one step each time, on the back from last step I'm getting this error in Browser console (All browsers, Chrome and Firefox). It is happening just on back from last step, it doesn't happen if I try to go back from other steps ( ex: from 3rd to 2nd).

TypeError: Cannot read property 'length' of undefined

    at callBackKeyUp (eval at <anonymous> (_osjs.js?9_1_601_0:18), <anonymous>:18:61)

    at Arguments.bindEvents (eval at <anonymous> (_osjs.js?9_1_601_0:18), <anonymous>:11:13)

    at OsAjaxBackendXHR.OnAfterAjaxRequest (_osjs.js?9_1_601_0:65)

    at eval (eval at <anonymous> (_osjs.js?9_1_601_0:18), <anonymous>:2:15)

    at eval (<anonymous>)

    at _osjs.js?9_1_601_0:18

    at Function.globalEval (_osjs.js?9_1_601_0:18)

    at OsEvaluateUserJavaScript (_osjs.js?9_1_601_0:5)

    at OsExecuteJSONUpdate (_osjs.js?9_1_601_0:5)

    at OsExecuteNextJSONUpdate (_osjs.js?9_1_601_0:5)

Hi Kovan,

Can you share the OML with this behavior, so I can check it out?