diff --git a/CHANGELOG.md b/CHANGELOG.md
index a141cfa2c..061ccf365 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog Fab-manager
 
 - Fix a bug: missing hour format for norwegian
+- Fix a bug: prepaid hours not used if bought slot is longer than remaining prepaid time
 
 ## v5.1.2 2021 July 2
 
diff --git a/app/models/cart_item/reservation.rb b/app/models/cart_item/reservation.rb
index 790c8a86d..3474eb87a 100644
--- a/app/models/cart_item/reservation.rb
+++ b/app/models/cart_item/reservation.rb
@@ -86,7 +86,7 @@ class CartItem::Reservation < CartItem::BaseItem
                    slot_rate
                  end
     # subtract free minutes from prepaid packs
-    if real_price.positive? && options[:prepaid][:minutes] >= slot_minutes
+    if real_price.positive? && options[:prepaid][:minutes] >= 0
       consumed = slot_minutes
       consumed = options[:prepaid][:minutes] if slot_minutes > options[:prepaid][:minutes]
       real_price = (slot_minutes - consumed) * (slot_rate / MINUTES_PER_HOUR)