diff --git a/pinaxcon/templatetags/nbpy_tags.py b/pinaxcon/templatetags/nbpy_tags.py
index d360555..c99050d 100644
--- a/pinaxcon/templatetags/nbpy_tags.py
+++ b/pinaxcon/templatetags/nbpy_tags.py
@@ -38,7 +38,7 @@ def donation_income(context, invoice):
     cheapest_ticket = tickets.order_by('price').first()
     ticket_rbi = nonvat_price(cheapest_ticket)
     return sum(
-        (nonvat_price(ticket) - ticket_rbi
+        (max(nonvat_price(ticket) - ticket_rbi, 0)
          for ticket in invoice.lineitem_set.filter(product__in=tickets)),
         Decimal(0)).quantize(CENTS_QUANT)