diff --git a/pretix_servicefees/signals.py b/pretix_servicefees/signals.py
index cecbc6e..974b2b4 100644
--- a/pretix_servicefees/signals.py
+++ b/pretix_servicefees/signals.py
@@ -35,7 +35,10 @@ def get_fees(event, total, invoice_address, mod=''):
     if mod and fee_percent is None:
         fee_percent = event.settings.get('service_fee_percent', as_type=Decimal)
 
-    if fee_abs and fee_percent and total != Decimal('0.00'):
+    fee_abs = Decimal("0") if fee_abs is None else fee_abs
+    fee_percent = Decimal("0") if fee_percent is None else fee_percent
+
+    if (fee_abs or fee_percent) and total != Decimal('0.00'):
         fee = round_decimal(fee_abs + total * (fee_percent / 100), event.currency)
         tax_rule = event.settings.tax_rate_default or TaxRule.zero()
         if tax_rule.tax_applicable(invoice_address):