Calculating Networkdays if it's blank in Power Query

Hi Everyone,

I would to calculate networkdays between the [Start] and [End]. the formulla went well with 

Networkdays([Start],[End],Holiday)

but in case [End] = null 

I've tried to replace null with today() [similar like excel] by writing 

Networkdays([Start],if [End] = null then DateTime.LocalNow else [End],Holiday)

Error show up as it seems wrong format. I've tried to put Date.From as well but it isn't work 

Networkdays([Start],if [End] = null then Date.From(DateTime.LocalNow) else [End],Holiday)

Your advice much be appreciated. Thank you so much in advance.