diff --git a/src/libexpr-c/nix_api_expr_internal.h b/src/libexpr-c/nix_api_expr_internal.h
index 2b066ecff..b50a51347 100644
--- a/src/libexpr-c/nix_api_expr_internal.h
+++ b/src/libexpr-c/nix_api_expr_internal.h
@@ -5,32 +5,6 @@
 #include "attr-set.hh"
 #include "nix_api_value.h"
 
-class CListBuilder
-{
-private:
-    std::vector<nix::Value *> values;
-
-public:
-    CListBuilder(size_t capacity)
-    {
-        values.reserve(capacity);
-    }
-
-    void push_back(nix::Value * value)
-    {
-        values.push_back(value);
-    }
-
-    Value * finish(nix::EvalState * state, nix::Value * list)
-    {
-        state->mkList(*list, values.size());
-        for (size_t n = 0; n < list->listSize(); ++n) {
-            list->listElems()[n] = values[n];
-        }
-        return list;
-    }
-};
-
 struct EvalState
 {
     nix::EvalState state;
@@ -43,7 +17,7 @@ struct BindingsBuilder
 
 struct ListBuilder
 {
-    CListBuilder builder;
+    nix::ListBuilder builder;
 };
 
 struct nix_string_return
diff --git a/src/libexpr-c/nix_api_value.cc b/src/libexpr-c/nix_api_value.cc
index 1faf05611..26ae194d9 100644
--- a/src/libexpr-c/nix_api_value.cc
+++ b/src/libexpr-c/nix_api_value.cc
@@ -263,7 +263,8 @@ Value * nix_get_list_byidx(nix_c_context * context, const Value * value, EvalSta
         assert(v.type() == nix::nList);
         auto * p = v.listElems()[ix];
         nix_gc_incref(nullptr, p);
-        state->state.forceValue(*p, nix::noPos);
+        if (p != nullptr)
+            state->state.forceValue(*p, nix::noPos);
         return (Value *) p;
     }
     NIXC_CATCH_ERRS_NULL
@@ -417,7 +418,7 @@ ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state,
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        auto builder = CListBuilder(capacity);
+        auto builder = state->state.buildList(capacity);
         return new
 #if HAVE_BOEHMGC
             (NoGC)
@@ -427,20 +428,21 @@ ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state,
     NIXC_CATCH_ERRS_NULL
 }
 
-nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, Value * value)
+nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, unsigned int index, Value * value)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        list_builder->builder.push_back((nix::Value *) value);
+        auto & e = check_value_not_null(value);
+        list_builder->builder[index] = &e;
     }
     NIXC_CATCH_ERRS
 }
 
-void nix_list_builder_free(ListBuilder * bb)
+void nix_list_builder_free(ListBuilder * list_builder)
 {
 #if HAVE_BOEHMGC
-    GC_FREE(bb);
+    GC_FREE(list_builder);
 #else
     delete bb;
 #endif
@@ -452,7 +454,7 @@ nix_err nix_make_list(nix_c_context * context, EvalState * s, ListBuilder * list
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_not_null(value);
-        list_builder->builder.finish(&(s->state), &v);
+        v.mkList(list_builder->builder);
     }
     NIXC_CATCH_ERRS
 }
diff --git a/src/libexpr-c/nix_api_value.h b/src/libexpr-c/nix_api_value.h
index b7115c27d..d80414fe8 100644
--- a/src/libexpr-c/nix_api_value.h
+++ b/src/libexpr-c/nix_api_value.h
@@ -360,17 +360,18 @@ ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state,
 /** @brief Insert bindings into a builder
  * @param[out] context Optional, stores error information
  * @param[in] list_builder ListBuilder to insert into
+ * @param[in] index index to manipulate
  * @param[in] value value to insert
  * @return error code, NIX_OK on success.
  */
-nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, Value * value);
+nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, unsigned int index, Value * value);
 
 /** @brief Free a list builder
  *
  * Does not fail.
  * @param[in] builder the builder to free
  */
-void nix_list_builder_free(ListBuilder * builder);
+void nix_list_builder_free(ListBuilder * list_builder);
 
 /** @brief Create an attribute set from a bindings builder
  * @param[out] context Optional, stores error information
diff --git a/tests/unit/libexpr/nix_api_value.cc b/tests/unit/libexpr/nix_api_value.cc
index ac28526c8..fd7c91a7d 100644
--- a/tests/unit/libexpr/nix_api_value.cc
+++ b/tests/unit/libexpr/nix_api_value.cc
@@ -77,12 +77,13 @@ TEST_F(nix_api_expr_test, nix_build_and_init_list)
 
     Value * intValue = nix_alloc_value(nullptr, state);
     nix_init_int(nullptr, intValue, 42);
-    nix_list_builder_insert(nullptr, builder, intValue);
+    nix_list_builder_insert(nullptr, builder, 0, intValue);
     nix_make_list(nullptr, state, builder, value);
     nix_list_builder_free(builder);
 
     ASSERT_EQ(42, nix_get_int(nullptr, nix_get_list_byidx(nullptr, value, state, 0)));
-    ASSERT_EQ(1, nix_get_list_size(nullptr, value));
+    ASSERT_EQ(nullptr, nix_get_list_byidx(nullptr, value, state, 1));
+    ASSERT_EQ(10, nix_get_list_size(nullptr, value));
 
     ASSERT_STREQ("a list", nix_get_typename(nullptr, value));
     ASSERT_EQ(NIX_TYPE_LIST, nix_get_type(nullptr, value));