If initial is something like

{count : 0L}

and the reduce is out.count++, you end up with NaN:

{ "retval" : [

{ "tag" : "A" , "count" : NaN}

,

{ "tag" : "B" , "count" : NaN}

,

{ "tag" : "C" , "count" : NaN}

] , "count" : 5.0 , "keys" : 3 , "ok" : 1.0}