diff --git a/doc/permutation_iterator_ref.rst b/doc/permutation_iterator_ref.rst
index 6972cb3..5489b09 100644
--- a/doc/permutation_iterator_ref.rst
+++ b/doc/permutation_iterator_ref.rst
@@ -64,9 +64,10 @@ If ``IndexIterator`` models Random Access Traversal Iterator and
 ``ElementIterator`` models Readable Lvalue Iterator then
 ``permutation_iterator`` models Random Access Iterator.
 
-``permutation_iterator<X, I1, V1, C2, R1, D1>`` is interoperable
-with ``permutation_iterator<Y, I2, V2, C2, R2, D2>`` if and only if
-``X`` is interoperable with ``Y``.
+``permutation_iterator<E1, X, V1, C2, R1, D1>`` is interoperable
+with ``permutation_iterator<E2, Y, V2, C2, R2, D2>`` if and only if
+``X`` is interoperable with ``Y`` and ``E1`` is convertible
+to ``E2``.
 
 
 ``permutation_iterator`` operations