diff --git a/mis_builder/models/mis_builder.py b/mis_builder/models/mis_builder.py
index 56f9b964..3e4cf638 100644
--- a/mis_builder/models/mis_builder.py
+++ b/mis_builder/models/mis_builder.py
@@ -144,7 +144,7 @@ class mis_report_kpi(orm.Model):
         res = {}
         if name and not _is_valid_python_var(name):
             res['warning'] = {
-                'title': 'Invalid name',
+                'title': 'Invalid name %s' % name,
                 'message': 'The name must be a valid python identifier'}
         return res
 
@@ -578,7 +578,7 @@ class mis_report_instance_period(orm.Model):
                     kpi_val = None
                     kpi_val_rendered = '#DIV/0'
                     kpi_val_comment += '\n\n%s' % (traceback.format_exc(),)
-                except ValueError:
+                except NameError:
                     recompute_queue.append(kpi)
                     kpi_val = None
                     kpi_val_rendered = '#ERR'
diff --git a/mis_builder/views/mis_builder.xml b/mis_builder/views/mis_builder.xml
index f93e1fcd..78178295 100644
--- a/mis_builder/views/mis_builder.xml
+++ b/mis_builder/views/mis_builder.xml
@@ -58,6 +58,33 @@
                             </tree>
                         </field>
                     </group>
+                    <group col="2" string="Legend (for expression)">
+                        <group>
+                            <label colspan="2" string="Expressions are of the form &lt;field&gt;&lt;mode&gt;[accounts][optional move line domain]"/>
+                            <label colspan="2" string="* Value for 'field' are :"/>
+                            <group>
+                                <label colspan="2" string="bal for balance of account"/>
+                                <label colspan="2" string="crd for credit of account"/>
+                                <label colspan="2" string="deb for debit of account"/>
+                            </group>
+                            <label colspan="2" string="* Value for 'mode' are :"/>
+                            <group>
+                                <label colspan="2" string="nothing or p for moves over period"/>
+                                <label colspan="2" string="i for for initial balance"/>
+                                <label colspan="2" string="e for ending balance"/>
+                            </group>
+                            <label colspan="2" string="* 'accounts' is a list of accounts, possibly containing %% wildcards"/>
+                            <label colspan="2" string="* 'optional move line domain' is an optional filters on eg analytic accounts or journal"/>
+                       </group>
+                       <group>
+                            <label colspan="2" string="Examples"/>
+                            <group>
+                                <label colspan="2" string="* bal[70]: balance of moves on account 70 over the period (it is the same as balp[70]);"/>
+                                <label colspan="2" string="* bali[70,60]: initial balance of accounts 70 and 60;"/>
+                                <label colspan="2" string="* bale[1%%]: balance of accounts starting with 1 at end of period."/>
+                            </group>
+                        </group>
+                    </group>
                 </sheet>
                 </form>
             </field>