diff --git a/read-cache.c b/read-cache.c
index 1e9a50c6c7..b36838e74b 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2373,6 +2373,7 @@ int discard_index(struct index_state *istate)
 	cache_tree_free(&(istate->cache_tree));
 	istate->initialized = 0;
 	istate->fsmonitor_has_run_once = 0;
+	FREE_AND_NULL(istate->fsmonitor_last_update);
 	FREE_AND_NULL(istate->cache);
 	istate->cache_alloc = 0;
 	discard_split_index(istate);
diff --git a/unpack-trees.c b/unpack-trees.c
index eb8fcda31b..9298fe1d9b 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1612,8 +1612,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->merge_size = len;
 	mark_all_ce_unused(o->src_index);
 
-	if (o->src_index->fsmonitor_last_update)
-		o->result.fsmonitor_last_update = o->src_index->fsmonitor_last_update;
+	o->result.fsmonitor_last_update =
+		xstrdup_or_null(o->src_index->fsmonitor_last_update);
 
 	/*
 	 * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries