Inconsistent implicit type conversions in Swift

gistfile1.txt

Welcome to Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1). Type :help for assistance.
  1> let an_int: Int = 3
an_int: Int = 3
  2> an_int is Int?
$R0: Bool = true
  3> [an_int] is [Int?]
$R1: Bool = true
  4> let list_of_ints = [an_int]
list_of_ints: [Int] = 1 value {
  [0] = 3
}
  5> list_of_ints is [Int?]
error: repl.swift:5:14: error: 'Int?' is not a subtype of 'Int'
list_of_ints is [Int?]
~~~~~~~~~~~~~^~~~~~~~~