diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 8582df5..f1af9a1 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -1,6 +1,8 @@
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_EXTENSIONS OFF)
 
+option(TEST_WITH_VALGRIND_MEMCHECK "Also run tests with valgrind / memcheck" ON)
+
 add_library(apfl
     alloc.c
     bytecode.c
@@ -39,6 +41,10 @@ function(unittest name moresources)
     add_executable(${name} test.h ${name}.c ${moresources})
     target_link_libraries(${name} PUBLIC apfl)
     add_test(NAME ${name} COMMAND ${name})
+    if(${TEST_WITH_VALGRIND_MEMCHECK})
+        add_test(NAME "valgrind_memcheck__${name}" COMMAND valgrind --tool=memcheck --leak-check=full -q --error-exitcode=99 $<TARGET_FILE:${name}>)
+    endif()
+
 endfunction()
 
 unittest(tokenizer_test "")
@@ -49,6 +55,9 @@ unittest(strings_test "")
 
 function(functionaltest name)
     add_test(NAME "functionaltest_${name}" COMMAND functional-test-runner ${CMAKE_SOURCE_DIR}/src/functional-tests/${name}.at)
+    if(${TEST_WITH_VALGRIND_MEMCHECK})
+        add_test(NAME "valgrind_memcheck__functionaltest_${name}" COMMAND valgrind --tool=memcheck --leak-check=full -q --error-exitcode=99 $<TARGET_FILE:functional-test-runner> ${CMAKE_SOURCE_DIR}/src/functional-tests/${name}.at)
+    endif()
 endfunction()
 
 functionaltest("hello-world")
diff --git a/src/apfl.h b/src/apfl.h
index e4382f9..7efe706 100644
--- a/src/apfl.h
+++ b/src/apfl.h
@@ -692,6 +692,8 @@ void apfl_get_member(apfl_ctx, apfl_stackidx container, apfl_stackidx k);
 bool apfl_get_member_if_exists(apfl_ctx, apfl_stackidx container, apfl_stackidx k);
 // Get a value from a list and push it on the stack. The list will stay on the stack.
 void apfl_get_list_member_by_index(apfl_ctx, apfl_stackidx list, size_t index_in_list);
+// Pop a value from the stack and replace the index_in_list-th entry in list with it. The index must already exist.
+void apfl_set_list_member_by_index(apfl_ctx, apfl_stackidx list, size_t index_in_list, apfl_stackidx value);
 // Get the length of a string/list/dict. The value stays on the stack,
 size_t apfl_len(apfl_ctx, apfl_stackidx);
 // Get a string view into a APFL_VALUE_STRING value. The value stays on the stack.
diff --git a/src/context.c b/src/context.c
index bddb207..99c304f 100644
--- a/src/context.c
+++ b/src/context.c
@@ -1176,8 +1176,42 @@ apfl_get_list_member_by_index(
     }
 
     *value = apfl_value_set_cow_flag(list.list->items[index]);
+}
 
-    return;
+void
+apfl_set_list_member_by_index(
+    apfl_ctx ctx,
+    apfl_stackidx list_index,
+    size_t index_in_list,
+    apfl_stackidx value_index
+) {
+    struct apfl_value *list = stack_get_pointer(ctx, list_index);
+    if (list == NULL) {
+        apfl_raise_invalid_stackidx(ctx);
+    }
+
+    if (list->type != VALUE_LIST) {
+        apfl_raise_const_error(ctx, APFL_RESULT_ERR, apfl_messages.not_a_list);
+    }
+
+    if (index_in_list >= list->list->len) {
+        apfl_raise_const_error(ctx, APFL_RESULT_ERR, apfl_messages.key_doesnt_exist);
+    }
+
+    struct apfl_value value = apfl_stack_must_get(ctx, value_index);
+
+    if (!apfl_list_splice(
+        &ctx->gc,
+        &list->list,
+        index_in_list,
+        1,
+        &value,
+        1
+    )) {
+        apfl_raise_alloc_error(ctx);
+    }
+
+    apfl_drop(ctx, value_index);
 }
 
 size_t
@@ -1283,45 +1317,28 @@ apfl_join_strings(apfl_ctx ctx, apfl_stackidx glue, apfl_stackidx parts)
     parts = -2;
     glue = -1;
 
-    struct apfl_value *parts_val = stack_get_pointer(ctx, parts);
-    assert(parts_val != NULL /* Should not be null, we moved it to a known position on the stack */);
-
-    if (parts_val->type != VALUE_LIST) {
+    if (apfl_get_type(ctx, parts) != APFL_VALUE_LIST) {
         apfl_raise_const_error(ctx, APFL_RESULT_ERR, apfl_messages.not_a_list);
     }
+    size_t parts_len = apfl_len(ctx, parts);
 
     apfl_tostring(ctx, glue);
     struct apfl_string_view glue_sv = apfl_get_string(ctx, glue);
 
-    struct list_header **parts_list = &parts_val->list;
-
     size_t total_length = 0;
 
-    for (size_t i = 0; i < (*parts_list)->len; i++) {
+    for (size_t i = 0; i < parts_len; i++) {
         // Convert values to strings, if necessary
-        if (!VALUE_IS_A((*parts_list)->items[i], APFL_VALUE_STRING)) {
-            apfl_stack_must_push(ctx, (*parts_list)->items[i]);
+        apfl_get_list_member_by_index(ctx, parts, i);
+        if (apfl_get_type(ctx, -1) == APFL_VALUE_STRING) {
+            total_length += apfl_get_string(ctx, -1).len;
+            apfl_drop(ctx, -1);
+        } else {
             apfl_tostring(ctx, -1);
-
-            if (!apfl_list_splice(
-                &ctx->gc,
-                parts_list,
-                i,
-                1,
-                stack_get_pointer(ctx, -1),
-                1
-            )) {
-                goto error;
-            }
-
-            apfl_stack_drop(ctx, -1);
+            total_length += apfl_get_string(ctx, -1).len;
+            apfl_set_list_member_by_index(ctx, -3, i, -1);
         }
 
-        struct apfl_string_view part_sv;
-        assert(get_string_view_of_value(&part_sv, (*parts_list)->items[i]) /* Value should be a string at this point */);
-
-        total_length += part_sv.len;
-
         if (i > 0) {
             total_length += glue_sv.len;
         }
@@ -1332,19 +1349,20 @@ apfl_join_strings(apfl_ctx ctx, apfl_stackidx glue, apfl_stackidx parts)
         goto error;
     }
 
-    for (size_t i = 0; i < (*parts_list)->len; i++) {
+    for (size_t i = 0; i < parts_len; i++) {
         if (i > 0) {
             if (!apfl_string_builder_append(&sb, glue_sv)) {
                 goto error;
             }
         }
 
-        struct apfl_string_view part_sv;
-        assert(get_string_view_of_value(&part_sv, (*parts_list)->items[i]) /* Value should be a string at this point */);
+        apfl_get_list_member_by_index(ctx, parts, i);
 
-        if (!apfl_string_builder_append(&sb, part_sv)) {
+        if (!apfl_string_builder_append(&sb, apfl_get_string(ctx, -1))) {
             goto error;
         }
+
+        apfl_drop(ctx, -1);
     }