diff --git a/src/pg/sql/25_optimization.sql b/src/pg/sql/25_optimization.sql
index be1d065..42c892f 100644
--- a/src/pg/sql/25_optimization.sql
+++ b/src/pg/sql/25_optimization.sql
@@ -12,10 +12,13 @@ RETURNS table(drain_id bigint, source_id int, cost numeric) AS $$
 
 from crankshaft.optimization import Optim
 
-params = {'waste_per_person': waste_per_person,
-          'recycle_rate': recycle_rate,
-          'dist_rate': dist_rate,
-          'dist_threshold': dist_threshold}
+def cast_val(val):
+    return float(val) if val is not None else None
+
+params = {'waste_per_person': cast_val(waste_per_person),
+          'recycle_rate': cast_val(recycle_rate), 
+          'dist_rate': cast_val(dist_rate),
+          'dist_threshold': cast_val(dist_threshold)}
 
 optim = Optim(drain, source, drain_capacity, source_production, marginal_cost,
               **params)
diff --git a/src/py/crankshaft/crankshaft/optimization/optim.py b/src/py/crankshaft/crankshaft/optimization/optim.py
index 7bcab7a..d32ece6 100644
--- a/src/py/crankshaft/crankshaft/optimization/optim.py
+++ b/src/py/crankshaft/crankshaft/optimization/optim.py
@@ -39,10 +39,9 @@ class Optim(object):
             'marginal_cost': self.data_provider.get_column(drain_table,
                                                            marginal_column),
             'distance': self.data_provider.get_pairwise_distances(source_table,
-                                                                  drain_table),
-            'cost': self.calc_cost()
-        }
-
+                                                                  drain_table)
+            }
+        self.model_data['cost'] = self.calc_cost()
         # database ids
         self.ids = {
             'drain': self.data_provider.get_column(drain_table,
@@ -74,8 +73,8 @@ class Optim(object):
                 self.model_params['amount_per_unit'] < 0):
             raise ValueError("`amount_per_unit` must be greater than zero.")
 
-        if (self.model_params['dist_threshold'] is None or
-                self.model_params['dist_threshold'] < 0):
+        if (self.model_params['dist_threshold'] <= 0 and
+                self.model_params['dist_threshold'] is not None):
             raise ValueError("`dist_threshold` must be greater than zero")
 
         if (self.model_params['dist_cost'] is None or