diff --git a/lib/api/v3/custom_fields/hierarchy/items_api.rb b/lib/api/v3/custom_fields/hierarchy/items_api.rb
index 7907839f4b0b..101e6fca6008 100644
--- a/lib/api/v3/custom_fields/hierarchy/items_api.rb
+++ b/lib/api/v3/custom_fields/hierarchy/items_api.rb
@@ -38,7 +38,10 @@ def flatten_tree_hash(hash)
               flat_list = []
               queue = [hash]
 
+              # From the service we get a hashed tree like this:
               # {:a => {:b => {:c1 => {:d1 => {}}, :c2 => {:d2 => {}}}, :b2 => {}}}
+              # We flatten it depth first to this result list:
+              # [:a, :b, :c1, :d1, :c2, :d2, :b2]
 
               while queue.any?
                 current = queue.shift
@@ -60,10 +63,20 @@ def item_list(query)
               start_item = ::CustomField::Hierarchy::Item.find_by(id: validation[:parent]) || hierarchy_root
               depth = validation[:depth] || -1
 
+              flat_tree(start_item, depth)
+            end
+
+            def flat_tree(item, depth)
               sub_tree = ::CustomFields::Hierarchy::HierarchicalItemService
                            .new
-                           .hashed_subtree(item: start_item, depth:)
-                           .value!
+                           .hashed_subtree(item:, depth:)
+                           .either(
+                             ->(value) { value },
+                             ->(error) do
+                               msg = "#{I18n.t('api_v3.errors.code_500')} #{error}"
+                               raise ::API::Errors::InternalError.new(msg)
+                             end
+                           )
 
               flatten_tree_hash(sub_tree)
             end