diff --git a/mis_builder/__openerp__.py b/mis_builder/__openerp__.py
old mode 100644
new mode 100755
diff --git a/mis_builder/models/accounting_none.py b/mis_builder/models/accounting_none.py
new file mode 100755
index 00000000..bcf15ecb
--- /dev/null
+++ b/mis_builder/models/accounting_none.py
@@ -0,0 +1,136 @@
+# -*- coding: utf-8 -*-
+
+
+"""
+Provides the AccountingNone singleton
+
+AccountingNone is a null value that dissolves in basic arithmetic operations,
+as illustrated in the examples below
+
+>>> 1 + 1
+2
+>>> 1 + AccountingNone
+1
+>>> AccountingNone + 1
+1
+>>> AccountingNone + None
+AccountingNone
+>>> +AccountingNone
+AccountingNone
+>>> -AccountingNone
+AccountingNone
+>>> -(AccountingNone)
+AccountingNone
+>>> AccountingNone - 1
+-1
+>>> 1 - AccountingNone
+1
+>>> AccountingNone - None
+AccountingNone
+>>> AccountingNone / 2
+0.0
+>>> 2 / AccountingNone
+Traceback (most recent call last):
+ ...
+ZeroDivisionError
+>>> AccountingNone / AccountingNone
+AccountingNone
+>>> AccountingNone // 2
+0.0
+>>> 2 // AccountingNone
+Traceback (most recent call last):
+ ...
+ZeroDivisionError
+>>> AccountingNone // AccountingNone
+AccountingNone
+>>> AccountingNone * 2
+0.0
+>>> 2 * AccountingNone
+0.0
+>>> AccountingNone * AccountingNone
+AccountingNone
+>>> AccountingNone * None
+AccountingNone
+"""
+
+
+class AccountingNoneType(object):
+
+    def __add__(self, other):
+        if other is None:
+            return AccountingNone
+        return other
+
+    __radd__ = __add__
+
+    def __sub__(self, other):
+        if other is None:
+            return AccountingNone
+        return -other
+
+    def __rsub__(self, other):
+        if other is None:
+            return AccountingNone
+        return other
+
+    def __iadd__(self, other):
+        if other is None:
+            return AccountingNone
+        return other
+
+    def __isub__(self, other):
+        if other is None:
+            return AccountingNone
+        return -other
+
+    def __pos__(self):
+        return self
+
+    def __neg__(self):
+        return self
+
+    def __floordiv__(self, other):
+        """
+        Overload of the // operator
+        """
+        if other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __rfloordiv__(self, other):
+        raise ZeroDivisionError
+
+    def __truediv__(self, other):
+        """
+        Overload of the / operator
+        """
+        if other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __rtruediv__(self, other):
+        raise ZeroDivisionError
+
+    def __mul__(self, other):
+        if other is None or other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __rmul__(self, other):
+        if other is None or other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __repr__(self):
+        return 'AccountingNone'
+
+    def __unicode__(self):
+        return ''
+
+
+AccountingNone = AccountingNoneType()
+
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
diff --git a/mis_builder/models/aep.py b/mis_builder/models/aep.py
old mode 100644
new mode 100755
index 4267c296..cbf63fea
--- a/mis_builder/models/aep.py
+++ b/mis_builder/models/aep.py
@@ -8,6 +8,8 @@ from collections import defaultdict
 from openerp.exceptions import Warning as UserError
 from openerp.models import expression
 from openerp.tools.safe_eval import safe_eval
+from openerp.tools.translate import _
+from .accounting_none import AccountingNone
 
 MODE_VARIATION = 'p'
 MODE_INITIAL = 'i'
@@ -228,12 +230,13 @@ class AccountingExpressionProcessor(object):
             field, mode, account_codes, domain = self._parse_match_object(mo)
             key = (domain, mode)
             account_ids_data = self._data[key]
-            v = 0.0
+            v = AccountingNone
             for account_code in account_codes:
                 account_ids = self._account_ids_by_code[account_code]
                 for account_id in account_ids:
                     debit, credit = \
-                        account_ids_data.get(account_id, (0.0, 0.0))
+                        account_ids_data.get(account_id,
+                                             (AccountingNone, AccountingNone))
                     if field == 'bal':
                         v += debit - credit
                     elif field == 'deb':
diff --git a/mis_builder/models/mis_builder.py b/mis_builder/models/mis_builder.py
old mode 100644
new mode 100755
index 17bc7aee..30106461
--- a/mis_builder/models/mis_builder.py
+++ b/mis_builder/models/mis_builder.py
@@ -16,6 +16,7 @@ from openerp.tools.safe_eval import safe_eval
 
 from .aep import AccountingExpressionProcessor as AEP
 from .aggregate import _sum, _avg, _min, _max
+from .accounting_none import AccountingNone
 
 _logger = logging.getLogger(__name__)
 
@@ -142,8 +143,8 @@ class MisReportKpi(models.Model):
     def render(self, lang_id, value):
         """ render a KPI value as a unicode string, ready for display """
         assert len(self) == 1
-        if value is None:
-            return '#N/A'
+        if value is None or value is AccountingNone:
+            return ''
         elif self.type == 'num':
             return self._render_num(lang_id, value, self.divider,
                                     self.dp, self.prefix, self.suffix)
@@ -469,6 +470,7 @@ class MisReportInstancePeriod(models.Model):
             'max': _max,
             'len': len,
             'avg': _avg,
+            'AccountingNone': AccountingNone,
         }
 
         localdict.update(self._fetch_queries())
@@ -515,7 +517,7 @@ class MisReportInstancePeriod(models.Model):
                              AEP.has_account_var(kpi.expression))
 
                 res[kpi.name] = {
-                    'val': kpi_val,
+                    'val': None if kpi_val is AccountingNone else kpi_val,
                     'val_r': kpi_val_rendered,
                     'val_c': kpi_val_comment,
                     'style': kpi_style,