Scala does tail recursion optimisation at compile-time and the recursive function is transformed into a loop by the compiler I was wondering why the following solution says "Run Time Error"

@tailrec
def mingle(p : String, q : String, acc : String = "" ) : String = {
if(p.isEmpty) acc
else  mingle(p.tail,q.tail,acc+p.head+q.head)

}

Is it because of same reason ?