ios - Could not find an overload for 'subscript' that accepts the supplied arguments - When convert Swift 1.2 to 2 -

i'm converting project swift 1.2 2. following code:

func pagecontrolchanged(pagecontrol: uipagecontrol) {     // current , upcoming page numbers     let currenttutorialpage = (viewcontrollers[0] as! partystepviewcontroller).page     let upcomingtutorialpage = pagecontrol.currentpage      // direction moving in?     let direction: uipageviewcontrollernavigationdirection = upcomingtutorialpage < currenttutorialpage ? .reverse : .forward      // set new page, animated!     setviewcontrollers([partystepforpage(upcomingtutorialpage)],         direction: direction, animated: true, completion: nil) } 

gave me following error:

could not find overload 'subscript' accepts supplied arguments

for particular line of code:

let currenttutorialpage = (viewcontrollers[0] as! partystepviewcontroller).page 

what error message mean? how can fix it?

here links code:

• a zipfile of whole project, not converted swift 2
• partystepviewcontroller
• partypagedviewcontroller

the problem viewcontrollers is optional.

so have unwrap before can subscript it: viewcontrollers![0]