[R-sig-Geo] join count with a grid that has NA's

[Ronnie Babigumira]

Hi listers
I am trying to apply join counts stats however my grid has some NA's and so I get an error

 > joincount.multi(mygrid.f, nb2listw(mynb, style="B"))
Error in joincount.multi(mygrid.f, nb2listw(mynb, style="B")) :
         NA in factor

My feeling was that the NA's can be ignored but looking at ?joincount.multi I dont see any options for ignoring NA's. 
Any ideas on a work around this

Ronnie