diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 43939e415c583..0099149286e14 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -523,6 +523,9 @@ def _simple_new(cls, values, name=None, dtype=None, **kwargs):
                                  **kwargs)._ndarray_values
 
         if isinstance(values, (ABCSeries, cls)):
+            # Index._data must always be an ndarray.
+            # This is no-copy for when _values is an ndarray,
+            # which should be always at this point.
             values = np.asarray(values._values)
 
         result = object.__new__(cls)