Xcode Previews don't seem to work for nested directories

Code Block 
// -- Sources/Foo/RootView.swift --
import SwiftUI
struct RootView: View {
    var body: some View { Text("Hello, World!") }
}
// Works!
struct RootView_Previews: PreviewProvider {
    static var previews: some View { RootView() }
}

Code Block 
// -- Sources/Foo/Subviews/SubView.swift --
import SwiftUI
struct SubView: View {
    var body: some View { Text("Hello, World!") }
}
// Error!
// NoBuildableEntriesError: active scheme does not build this file
// Select a scheme that builds a target which contains the current file, or add this file to a target that is built by the current scheme.
struct SubView_Previews: PreviewProvider {
    static var previews: some View { SubView() }
}