diff --git a/commit-graph.c b/commit-graph.c
index 03e5a98796..edbb3a0f2c 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1193,7 +1193,9 @@ static int write_graph_chunk_generation_data(struct hashfile *f,
 
 	for (i = 0; i < ctx->commits.nr; i++) {
 		struct commit *c = ctx->commits.list[i];
-		timestamp_t offset = commit_graph_data_at(c)->generation - c->date;
+		timestamp_t offset;
+		repo_parse_commit(ctx->r, c);
+		offset = commit_graph_data_at(c)->generation - c->date;
 		display_progress(ctx->progress, ++ctx->progress_cnt);
 
 		if (offset > GENERATION_NUMBER_V2_OFFSET_MAX) {
@@ -1444,15 +1446,20 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 					_("Computing commit graph generation numbers"),
 					ctx->commits.nr);
 	for (i = 0; i < ctx->commits.nr; i++) {
-		uint32_t level = *topo_level_slab_at(ctx->topo_levels, ctx->commits.list[i]);
-		timestamp_t corrected_commit_date = commit_graph_data_at(ctx->commits.list[i])->generation;
+		struct commit *c = ctx->commits.list[i];
+		uint32_t level;
+		timestamp_t corrected_commit_date;
+
+		repo_parse_commit(ctx->r, c);
+		level = *topo_level_slab_at(ctx->topo_levels, c);
+		corrected_commit_date = commit_graph_data_at(c)->generation;
 
 		display_progress(ctx->progress, i + 1);
 		if (level != GENERATION_NUMBER_ZERO &&
 		    corrected_commit_date != GENERATION_NUMBER_ZERO)
 			continue;
 
-		commit_list_insert(ctx->commits.list[i], &list);
+		commit_list_insert(c, &list);
 		while (list) {
 			struct commit *current = list->item;
 			struct commit_list *parent;
@@ -1461,6 +1468,7 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 			timestamp_t max_corrected_commit_date = 0;
 
 			for (parent = current->parents; parent; parent = parent->next) {
+				repo_parse_commit(ctx->r, parent->item);
 				level = *topo_level_slab_at(ctx->topo_levels, parent->item);
 				corrected_commit_date = commit_graph_data_at(parent->item)->generation;
 
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index fa27df579a..2cf29f425a 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -446,6 +446,27 @@ test_expect_success 'warn on improper hash version' '
 	)
 '
 
+test_expect_failure 'lower layers have overflow chunk' '
+	cd "$TRASH_DIRECTORY/full" &&
+	UNIX_EPOCH_ZERO="@0 +0000" &&
+	FUTURE_DATE="@2147483646 +0000" &&
+	rm -f .git/objects/info/commit-graph &&
+	test_commit --date "$FUTURE_DATE" future-1 &&
+	test_commit --date "$UNIX_EPOCH_ZERO" old-1 &&
+	git commit-graph write --reachable &&
+	test_commit --date "$FUTURE_DATE" future-2 &&
+	test_commit --date "$UNIX_EPOCH_ZERO" old-2 &&
+	git commit-graph write --reachable --split=no-merge &&
+	test_commit extra &&
+	git commit-graph write --reachable --split=no-merge &&
+	git commit-graph write --reachable &&
+	graph_read_expect 16 "generation_data generation_data_overflow extra_edges" &&
+	mv .git/objects/info/commit-graph commit-graph-upgraded &&
+	git commit-graph write --reachable &&
+	graph_read_expect 16 "generation_data generation_data_overflow extra_edges" &&
+	test_cmp .git/objects/info/commit-graph commit-graph-upgraded
+'
+
 # the verify tests below expect the commit-graph to contain
 # exactly the commits reachable from the commits/8 branch.
 # If the file changes the set of commits in the list, then the