Swift. Type does not conform to protocol. The type and the protocol are the same

Question

I have a protocol:

protocol InnerObject
{
   init(record : NSManagedObject)
   init(pfObject : PFObject)
   func saveToRecord(aRecord: NSManagedObject)
   func toPFObject() -> PFObject
}

and I have an extension for an array:

extension Array where Element : InnerObject
{
    func pfObjects() -> [PFObject]
   {
       var pfObjects : [PFObject] = []

       for innerObject in self
       {
           let pfObject : PFObject = innerObject.toPFObject()
           pfObjects.append(pfObject)
       }

       return pfObjects
   }
}

and I'm trying to use it:

func saveObjects(aObjects : [InnerObject], withCompletion aCompletion : () -> (Bool, NSError))
{
    let array : [PFObject] = aObjects.pfObjects() // the error is here
}

but it produces an error:

Type 'InnerObject' does not conform to protocol 'InnerObject'

How do I resolve it?