diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index dfb4c87cfe59..38040fb53ea9 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -383,8 +383,8 @@ auto CarbonExternalASTSource::MapInstIdToClangDecl(
     auto* namespace_decl = clang::NamespaceDecl::Create(
         *ast_context_, &decl_context, false, clang::SourceLocation(),
         clang::SourceLocation(), identifier_info, nullptr, false);
-    auto result = scope_mapping.Insert(
-        static_cast<clang::DeclContext*>(namespace_decl), target_inst_id);
+    auto result = scope_mapping.Insert(namespace_decl->getPrimaryContext(),
+                                       target_inst_id);
     CARBON_CHECK(result.is_inserted(), "Inserting over an existing entry.");
     namespace_decl->setHasExternalVisibleStorage();
     return namespace_decl;
@@ -395,7 +395,8 @@ auto CarbonExternalASTSource::MapInstIdToClangDecl(
 auto CarbonExternalASTSource::FindExternalVisibleDeclsByName(
     const clang::DeclContext* decl_context, clang::DeclarationName decl_name,
     const clang::DeclContext* /*OriginalDC*/) -> bool {
-  auto decl_context_inst_id = scope_mapping.Lookup(decl_context);
+  auto decl_context_inst_id =
+      scope_mapping.Lookup(decl_context->getPrimaryContext());
 
   if (!decl_context_inst_id) {
     // If the context doesn't already have a mapping between C++ and Carbon,
@@ -420,9 +421,9 @@ auto CarbonExternalASTSource::FindExternalVisibleDeclsByName(
         clang::SourceLocation(), &ast_context.Idents.get(carbon_namespace_name),
         nullptr, false);
     carbon_cpp_namespace->setHasExternalVisibleStorage();
-    auto result = scope_mapping.Insert(
-        static_cast<clang::DeclContext*>(carbon_cpp_namespace),
-        SemIR::Namespace::PackageInstId);
+    auto result =
+        scope_mapping.Insert(carbon_cpp_namespace->getPrimaryContext(),
+                             SemIR::Namespace::PackageInstId);
     CARBON_CHECK(result.is_inserted(), "Inserting over an existing entry.");
     SetExternalVisibleDeclsForName(decl_context, decl_name,
                                    {carbon_cpp_namespace});