diff --git a/Foundation/String.swift b/Foundation/String.swift
index 053274c97a..4092a6bf2f 100644
--- a/Foundation/String.swift
+++ b/Foundation/String.swift
@@ -74,7 +74,10 @@ extension String {
   /// Return an `Index` corresponding to the given offset in our UTF-16
   /// representation.
   func _index(_ utf16Index: Int) -> Index {
-    return Index(_base: String.UnicodeScalarView.Index(utf16Index, _core))
+    return Index(
+      _base: String.UnicodeScalarView.Index(_position: utf16Index),
+      in: characters
+    )
   }
 
   /// Return a `Range<Index>` corresponding to the given `NSRange` of