diff --git a/www/conservancy/apps/supporter/views.py b/www/conservancy/apps/supporter/views.py
index cad15e8071a0db88f628cbdeebc4eb961275877b..0c92941d4cd3c05bf59d7c9f6a657215ec7622b8 100644
--- a/www/conservancy/apps/supporter/views.py
+++ b/www/conservancy/apps/supporter/views.py
@@ -1,7 +1,14 @@
 import conservancy
 
 def index(request):
-    partial_amount = conservancy.param_if_valid(request.GET, 'upgrade', 'upgrade_id', 0)
+    with conservancy.ParameterValidator(request.GET, 'upgrade_id') as validator:
+        try:
+            amount_param = float(request.GET['upgrade'])
+        except (KeyError, ValueError):
+            validator.fail()
+        else:
+            validator.validate('{.2f}'.format(amount_param))
+    partial_amount = amount_param if validator.valid else 0
     context = {
         'partial_amount': partial_amount,
         'minimum_amount': 120 - partial_amount,