diff --git a/mis_builder/models/accounting_none.py b/mis_builder/models/accounting_none.py
new file mode 100644
index 00000000..5249e540
--- /dev/null
+++ b/mis_builder/models/accounting_none.py
@@ -0,0 +1,187 @@
+# -*- coding: utf-8 -*-
+# © 2016 Thomas Binsfeld
+# © 2016 ACSONE SA/NV (<http://acsone.eu>)
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
+"""
+Provides the AccountingNone singleton.
+
+AccountingNone is a null value that dissolves in basic arithmetic operations,
+as illustrated in the examples below. In comparisons, AccountingNone behaves
+the same as zero.
+
+>>> 1 + 1
+2
+>>> 1 + AccountingNone
+1
+>>> AccountingNone + 1
+1
+>>> AccountingNone + None
+AccountingNone
+>>> +AccountingNone
+AccountingNone
+>>> -AccountingNone
+AccountingNone
+>>> -(AccountingNone)
+AccountingNone
+>>> AccountingNone - 1
+-1
+>>> 1 - AccountingNone
+1
+>>> abs(AccountingNone)
+AccountingNone
+>>> 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
+>>> None * AccountingNone
+AccountingNone
+>>> str(AccountingNone)
+''
+>>> bool(AccountingNone)
+False
+>>> AccountingNone > 0
+False
+>>> AccountingNone < 0
+False
+>>> AccountingNone < 1
+True
+>>> AccountingNone > 1
+False
+>>> 0 < AccountingNone
+False
+>>> 0 > AccountingNone
+False
+>>> 1 < AccountingNone
+False
+>>> 1 > AccountingNone
+True
+>>> AccountingNone == 0
+True
+>>> AccountingNone == 0.0
+True
+>>> AccountingNone == None
+True
+"""
+
+__all__ = ['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 __abs__(self):
+        return self
+
+    def __pos__(self):
+        return self
+
+    def __neg__(self):
+        return self
+
+    def __div__(self, other):
+        if other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __rdiv__(self, other):
+        raise ZeroDivisionError
+
+    def __floordiv__(self, other):
+        if other is AccountingNone:
+            return AccountingNone
+        return 0.0
+
+    def __rfloordiv__(self, other):
+        raise ZeroDivisionError
+
+    def __truediv__(self, other):
+        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
+
+    __rmul__ = __mul__
+
+    def __repr__(self):
+        return 'AccountingNone'
+
+    def __str__(self):
+        return ''
+
+    def __nonzero__(self):
+        return False
+
+    def __bool__(self):
+        return False
+
+    def __eq__(self, other):
+        return other == 0 or other is None or other is AccountingNone
+
+    def __lt__(self, other):
+        return 0 < other
+
+    def __gt__(self, other):
+        return 0 > other
+
+
+AccountingNone = AccountingNoneType()
+
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
diff --git a/mis_builder/models/aep.py b/mis_builder/models/aep.py
index ffa344ae..8562d863 100644
--- a/mis_builder/models/aep.py
+++ b/mis_builder/models/aep.py
@@ -9,6 +9,7 @@ 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'
@@ -348,12 +349,13 @@ 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
index 20271920..df44230e 100644
--- 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__)
 
@@ -140,8 +141,8 @@ 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)
@@ -153,31 +154,45 @@ class MisReportKpi(models.Model):
 
     def render_comparison(self, lang_id, value, base_value,
                           average_value, average_base_value):
-        """ render the comparison of two KPI values, ready for display """
+        """ render the comparison of two KPI values, ready for display
+
+        If the difference is 0, an empty string is returned.
+        """
         assert len(self) == 1
-        if value is None or base_value is None:
-            return ''
+        if value is None:
+            value = AccountingNone
+        if base_value is None:
+            base_value = AccountingNone
         if self.type == 'pct':
-            return self._render_num(
-                lang_id,
-                value - base_value,
-                0.01, self.dp, '', _('pp'), sign='+')
+            delta = value - base_value
+            if delta and round(delta, self.dp) != 0:
+                return self._render_num(
+                    lang_id,
+                    delta,
+                    0.01, self.dp, '', _('pp'),
+                    sign='+')
         elif self.type == 'num':
-            if average_value:
+            if value and average_value:
                 value = value / float(average_value)
-            if average_base_value:
+            if base_value and average_base_value:
                 base_value = base_value / float(average_base_value)
             if self.compare_method == 'diff':
-                return self._render_num(
-                    lang_id,
-                    value - base_value,
-                    self.divider, self.dp, self.prefix, self.suffix, sign='+')
-            elif self.compare_method == 'pct':
-                if round(base_value, self.dp) != 0:
+                delta = value - base_value
+                if delta and round(delta, self.dp) != 0:
                     return self._render_num(
                         lang_id,
-                        (value - base_value) / abs(base_value),
-                        0.01, self.dp, '', '%', sign='+')
+                        delta,
+                        self.divider, self.dp, self.prefix, self.suffix,
+                        sign='+')
+            elif self.compare_method == 'pct':
+                if base_value and round(base_value, self.dp) != 0:
+                    delta = (value - base_value) / abs(base_value)
+                    if delta and round(delta, self.dp) != 0:
+                        return self._render_num(
+                            lang_id,
+                            delta,
+                            0.01, self.dp, '', '%',
+                            sign='+')
         return ''
 
     def _render_num(self, lang_id, value, divider,
@@ -497,6 +512,7 @@ class MisReportInstancePeriod(models.Model):
             'max': _max,
             'len': len,
             'avg': _avg,
+            'AccountingNone': AccountingNone,
         }
 
         localdict.update(self._fetch_queries())
@@ -544,7 +560,7 @@ 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,