diff --git a/app/models/tarifs/group_minimum.rb b/app/models/tarifs/group_minimum.rb
index 269789561..e990bf460 100644
--- a/app/models/tarifs/group_minimum.rb
+++ b/app/models/tarifs/group_minimum.rb
@@ -55,7 +55,7 @@ def group_price(usage)
     end
 
     def group_used_units(usage)
-      usages_in_group(usage).sum(&:used_units)
+      usages_in_group(usage).sum { _1.used_units || 0 }
     end
 
     def minimum_prices_with_difference(usage) # rubocop:disable Metrics/AbcSize
diff --git a/spec/models/tarifs/group_minimum_spec.rb b/spec/models/tarifs/group_minimum_spec.rb
index 638fc6379..768160a40 100644
--- a/spec/models/tarifs/group_minimum_spec.rb
+++ b/spec/models/tarifs/group_minimum_spec.rb
@@ -53,9 +53,11 @@
   let(:tarif) { create(:tarif, type: described_class.sti_name, price_per_unit:, organisation:, tarif_group:) }
   let(:other_tarif1) { create(:tarif, price_per_unit:, organisation:, tarif_group:) }
   let(:other_tarif2) { create(:tarif, price_per_unit: 12, organisation:, tarif_group:) }
+  let(:other_tarif3) { create(:tarif, price_per_unit: 0, organisation:, tarif_group:) }
   let!(:usage) { create(:usage, booking:, tarif:) }
   let!(:other_usage1) { create(:usage, booking:, tarif: other_tarif1, used_units: 7) }
   let!(:other_usage2) { create(:usage, booking:, tarif: other_tarif2, used_units: 8) }
+  let!(:other_usage3) { create(:usage, booking:, tarif: other_tarif3, used_units: nil) }
 
   describe '#minimum_prices' do
     subject(:minimum_prices) { tarif.minimum_prices_with_difference(usage) }