Getting index or using a count inside .jade mixing

In trying to assign a "data-index" to an element, I'd like to give data-index the value the index value of the current item in items. The code below if what I have so far but it doesn't seem to work not sure if I'm actually doing it right. I just receive a gulp-jade error saying jade_mixins.partySection is not a function

mixin partySection(items)
- var count=0:
    each item in items
      .col-md-3.col-lg-3
        button.party-button(data-index=count data-section="Party")
- count++;

via J.Rem