diff --git a/modules/purchase_trade/tests/__init__.py b/modules/purchase_trade/tests/__init__.py
new file mode 100644
index 0000000..4effdfa
--- /dev/null
+++ b/modules/purchase_trade/tests/__init__.py
@@ -0,0 +1,2 @@
+# This file is part of Tryton.  The COPYRIGHT file at the top level of
+# this repository contains the full copyright notices and license terms.
diff --git a/modules/purchase_trade/tests/test_module.py b/modules/purchase_trade/tests/test_module.py
new file mode 100644
index 0000000..2f49993
--- /dev/null
+++ b/modules/purchase_trade/tests/test_module.py
@@ -0,0 +1,52 @@
+# This file is part of Tryton.  The COPYRIGHT file at the top level of
+# this repository contains the full copyright notices and license terms.
+
+from decimal import Decimal
+from unittest.mock import Mock, patch
+
+from trytond.pool import Pool
+from trytond.tests.test_tryton import ModuleTestCase, with_transaction
+
+
+class PurchaseTradeTestCase(ModuleTestCase):
+    'Test purchase_trade module'
+    module = 'purchase_trade'
+
+    @with_transaction()
+    def test_get_totals_without_rows(self):
+        'get_totals returns zeros when the query has no row'
+        Valuation = Pool().get('valuation.valuation')
+        cursor = Mock()
+        cursor.fetchone.return_value = None
+        connection = Mock()
+        connection.cursor.return_value = cursor
+
+        with patch(
+                'trytond.modules.purchase_trade.valuation.Transaction'
+                ) as Transaction, patch.object(
+                    Valuation, '__table__', return_value='valuation_valuation'):
+            Transaction.return_value.connection = connection
+
+            self.assertEqual(
+                Valuation.get_totals(), (Decimal(0), Decimal(0)))
+
+    @with_transaction()
+    def test_get_totals_without_previous_total(self):
+        'get_totals converts null variation to zero'
+        Valuation = Pool().get('valuation.valuation')
+        cursor = Mock()
+        cursor.fetchone.return_value = (Decimal('42.50'), None)
+        connection = Mock()
+        connection.cursor.return_value = cursor
+
+        with patch(
+                'trytond.modules.purchase_trade.valuation.Transaction'
+                ) as Transaction, patch.object(
+                    Valuation, '__table__', return_value='valuation_valuation'):
+            Transaction.return_value.connection = connection
+
+            self.assertEqual(
+                Valuation.get_totals(), (Decimal('42.50'), Decimal(0)))
+
+
+del ModuleTestCase
diff --git a/modules/purchase_trade/valuation.py b/modules/purchase_trade/valuation.py
index 3142d05..3dc7cf3 100644
--- a/modules/purchase_trade/valuation.py
+++ b/modules/purchase_trade/valuation.py
@@ -459,9 +459,13 @@ class Valuation(ValuationBase, ModelView):
         """
 
         cursor.execute(sql)
-        last_total, last_variation = cursor.fetchone()
+        row = cursor.fetchone()
+        if not row:
+            return Decimal(0), Decimal(0)
 
-        return last_total, last_variation
+        last_total, last_variation = row
+
+        return last_total or Decimal(0), last_variation or Decimal(0)
 
 class ValuationLine(ValuationBase, ModelView):
     "Last Valuation"