diff --git a/tests/olap.model b/tests/olap.model
index 126ba8c..8f33ab8 100644
--- a/tests/olap.model
+++ b/tests/olap.model
@@ -263,7 +263,7 @@
              "name": "bool",
              "type": "bool",
              "value": "\"field_bool\"",
-             "value_label": "(case when \"field_bool\" IS NOT NULL then 'Oui' else 'Non' end)"
+             "value_label": "(CASE WHEN \"field_bool\" IS NULL THEN NULL WHEN \"field_bool\" THEN 'Oui' ELSE 'Non' END)"
             }
          ],
          "fact_table" : "formdata_demande",
diff --git a/wcs_olap/feeder.py b/wcs_olap/feeder.py
index 90850d0..179d298 100644
--- a/wcs_olap/feeder.py
+++ b/wcs_olap/feeder.py
@@ -902,7 +902,11 @@ class WcsFormdefFeeder(object):
                     'label': field.label.lower(),
                     'type': 'bool',
                     'value': '"field_%s"' % field.varname,
-                    'value_label': '(case when "field_%s" IS NOT NULL then \'Oui\' else \'Non\' end)' % field.varname,
+                    'value_label': '(CASE WHEN "field_%(varname)s" IS NULL THEN NULL'
+                                   ' WHEN "field_%(varname)s" THEN \'Oui\''
+                                   ' ELSE \'Non\' END)' % {
+                        'varname': field.varname,
+                    },
                     'filter': True,
                 }
             elif field.type == 'string':