Summing the values of multiple Vectors in the session

In my gatling scenario, I need to check the session for a few entries that will be Vectors of numbers. I can get the Vectors if present, but when I attempt to add them using .sum I get a ClassCastException stating that java.lang.String can't be cast to java.lang.Integer

I've debugged by printing out the value retrieved from the session (Vector(100,200,300)), and even confirmed that the individual elements are Ints. However when I try to add any of them, either with something like values.sum or values(0)+values(1) I get the class cast exception

I'm setting values in the session with checks like

.check(jsonPath("$..payments..paymentAmount").findAll.optional.saveAs("payments"))
.check(jsonPath("$..receipts..receiptAmount").findAll.optional.saveAs("receipts"))

in my app these will always result in things like Vector(100, 200, 300) if the path was there

then later I want to sum all the values in these lists so I have the action

.exec(session => {
   def addAmounts(listNames: Array[String]): Int = {
          listNames.foldLeft(0)((acc, listName) => {
            session(listName).validate[Seq[Int]] match {
              case Success(ints) =>  ints.sum + acc
              case Failure(error) => acc
          }})
   }

   val transactionsTotal = addAmounts(Array("payments", "receipts"))
   session.set("total",transactionsTotal) 
}

As mentioned, this fails on the listName.sum + acc statement - since they're both Ints I'd expect there'd be no need to cast from a string The Failure case where nothing was stored from the check works fine

I think this is a scala type inference issue - I got it working by manually casting to Int before doing addition