diff --git a/SeQuant/core/index.cpp b/SeQuant/core/index.cpp
index 0c4da854b5..24efc88612 100644
--- a/SeQuant/core/index.cpp
+++ b/SeQuant/core/index.cpp
@@ -71,4 +71,9 @@ std::string Index::to_string() const {
   return sequant::to_string(this->label());
 }
 
+std::shared_ptr<const IndexSpaceRegistry>
+Index::obtain_default_index_registry() {
+  return get_default_context().index_space_registry();
+}
+
 }  // namespace sequant
diff --git a/SeQuant/core/index.hpp b/SeQuant/core/index.hpp
index fed1ffaf20..92b9869e59 100644
--- a/SeQuant/core/index.hpp
+++ b/SeQuant/core/index.hpp
@@ -6,8 +6,9 @@
 #define SEQUANT_INDEX_H
 
 #include <SeQuant/core/container.hpp>
-#include <SeQuant/core/context.hpp>
 #include <SeQuant/core/hash.hpp>
+#include <SeQuant/core/index_space_registry.hpp>
+#include <SeQuant/core/space.hpp>
 #include <SeQuant/core/tag.hpp>
 #include <SeQuant/core/utility/macros.hpp>
 #include <SeQuant/core/utility/string.hpp>
@@ -15,7 +16,6 @@
 
 #include <algorithm>
 #include <atomic>
-#include <charconv>
 #include <cstdint>
 #include <cwchar>
 #include <functional>
@@ -23,6 +23,7 @@
 #include <iostream>
 #include <iterator>
 #include <map>
+#include <memory>
 #include <mutex>
 #include <optional>
 #include <stdexcept>
@@ -328,12 +329,11 @@ class Index : public Taggable {
   /// the cost of slightly increased danger
   template <basic_string_convertible String>
   Index(String &&label)
-      : Index(
-            get_default_context().index_space_registry()
-                ? get_default_context().index_space_registry()->retrieve(label)
-                : IndexSpace{base_label(label), IndexSpace::Type::reserved,
-                             IndexSpace::QuantumNumbers::reserved},
-            to_ordinal(label), {}) {
+      : Index(obtain_default_index_registry()
+                  ? obtain_default_index_registry()->retrieve(label)
+                  : IndexSpace{base_label(label), IndexSpace::Type::reserved,
+                               IndexSpace::QuantumNumbers::reserved},
+              to_ordinal(label), {}) {
     check_nonreserved();
     if constexpr (std::is_same_v<String, std::wstring>) {
       label_ = std::move(label);
@@ -1043,6 +1043,8 @@ class Index : public Taggable {
     return i1_Q < i2_Q ? SO::less : SO::greater;
   }
 
+  static std::shared_ptr<const IndexSpaceRegistry> obtain_default_index_registry();
+
 };  // class Index
 
 inline const IndexSpace::Attr Index::default_space_attr{
diff --git a/SeQuant/core/options.cpp b/SeQuant/core/options.cpp
index acd7fbb2fd..597ec35a9c 100644
--- a/SeQuant/core/options.cpp
+++ b/SeQuant/core/options.cpp
@@ -45,9 +45,9 @@ CanonicalizeOptions CanonicalizeOptions::copy_and_set(
 }
 
 CanonicalizeOptions CanonicalizeOptions::copy_and_set(
-    std::optional<std::initializer_list<Index>> arg) const {
+    std::optional<std::vector<Index>> arg) const {
   auto result = *this;
-  result.named_indices = arg;
+  result.named_indices = std::move(arg);
   return result;
 }
 
diff --git a/SeQuant/core/options.hpp b/SeQuant/core/options.hpp
index 11b4f0f1b7..435394dd80 100644
--- a/SeQuant/core/options.hpp
+++ b/SeQuant/core/options.hpp
@@ -5,12 +5,14 @@
 #ifndef SEQUANT_CORE_OPTIONS_HPP
 #define SEQUANT_CORE_OPTIONS_HPP
 
+#include <SeQuant/core/index.hpp>
+
+#include <optional>
 #include <string>
+#include <vector>
 
 namespace sequant {
 
-class Index;
-
 /// canonicalization methods
 enum class CanonicalizationMethod {
   /// Enables use of expression topology in the canonicalization, may be
@@ -47,7 +49,7 @@ struct CanonicalizeOptions {
   /// specifies named indices; by default all indices that appear only once are
   /// deduced to be named, but this may be misleading if e.g. single
   /// summed-over dummy index appears in an expression
-  std::optional<std::initializer_list<Index>> named_indices = std::nullopt;
+  std::optional<std::vector<Index>> named_indices = std::nullopt;
   /// whether to ignore the labels of named indices. Setting
   /// to false will cause named indices to be treated as equivalent slots, which
   /// the result to be independent of their labels. This does not make sense in
@@ -57,8 +59,7 @@ struct CanonicalizeOptions {
 
   static CanonicalizeOptions default_options();
   CanonicalizeOptions copy_and_set(CanonicalizationMethod) const;
-  CanonicalizeOptions copy_and_set(
-      std::optional<std::initializer_list<Index>>) const;
+  CanonicalizeOptions copy_and_set(std::optional<std::vector<Index>>) const;
   CanonicalizeOptions copy_and_set(IgnoreNamedIndexLabel) const;
 
   friend constexpr bool operator==(const CanonicalizeOptions& a,
diff --git a/tests/unit/test_spin.cpp b/tests/unit/test_spin.cpp
index b3d1cb9983..4f720c78f6 100644
--- a/tests/unit/test_spin.cpp
+++ b/tests/unit/test_spin.cpp
@@ -632,6 +632,20 @@ SECTION("partial spintracing + S_maps = full spintracing") {
   //(canonicalized: -2 t{a_1,a_2;i_2,i_1}:N-C-S + 4 t{a_1,a_2;i_1,i_2}:N-C-S)
 }
 
+SECTION("external indices have to remain consistent") {
+  // Internally, closed_shell_spintrace performs canonicalization which can end
+  // up renaming external indices (due to the presence of the symmetrizer). This
+  // will cause an inconsistency with the explicitly provided external index
+  // names.
+  ExprPtr input = parse_expr(
+      L"A{p_8,p_9;a_8,a_9}:A-C-S * y{p_3,p_4;p_8,p_9}:A-C-S * "
+      L"g{i_3,a_8;p_3,p_4}:A-C-S * t{a_9;i_3}:A-C-S");
+  ExprPtr result =
+      closed_shell_spintrace(input, {{L"a_8", L"p_8"}, {L"a_9", L"p_9"}});
+
+  // REQUIRE_THAT(result, EquivalentTo(L"TODO"));
+}
+
 SECTION("Symmetrize expression") {
   {
     // g * t1 + g * t1