diff --git a/trytond/doc/topics/domain.rst b/trytond/doc/topics/domain.rst
index ba991ff298..e65a166f18 100644
--- a/trytond/doc/topics/domain.rst
+++ b/trytond/doc/topics/domain.rst
@@ -37,6 +37,10 @@ by this pattern::
 
     The number of *dots* in a clause is not limited.
 
+    .. note::
+      Negative operators match records for which any of the field before the
+      last dot is empty.
+
     .. warning::
         For :class:`trytond.model.fields.Reference`, an extra ending clause is
         needed to define the target model to join, for example::
diff --git a/trytond/trytond/model/fields/many2one.py b/trytond/trytond/model/fields/many2one.py
index 0e3a9a1816..eb73d46921 100644
--- a/trytond/trytond/model/fields/many2one.py
+++ b/trytond/trytond/model/fields/many2one.py
@@ -1,6 +1,6 @@
 # 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 sql import As, Column, Expression, Literal, Query, With
+from sql import As, Column, Expression, Literal, Null, Query, With
 from sql.aggregate import Max
 from sql.conditionals import Coalesce
 from sql.operators import Or
@@ -266,7 +266,7 @@ class Many2One(Field):
         target_domain = [(target_name,) + tuple(domain[1:])]
         if Target.estimated_count() < _subquery_threshold:
             query = Target.search(target_domain, order=[], query=True)
-            return column.in_(query)
+            expression = column.in_(query)
         else:
             rule_domain = Rule.domain_get(Target.__name__, mode='read')
             if rule_domain:
@@ -279,7 +279,9 @@ class Many2One(Field):
             target_table, _ = target_tables[None]
             _, expression = Target.search_domain(
                 target_domain, tables=target_tables)
-            return expression
+        if operator.startswith('not') or operator == '!=':
+            expression |= column == Null
+        return expression
 
     @order_method
     def convert_order(self, name, tables, Model):
diff --git a/trytond/trytond/model/fields/reference.py b/trytond/trytond/model/fields/reference.py
index 42865c69fd..520b4cf38e 100644
--- a/trytond/trytond/model/fields/reference.py
+++ b/trytond/trytond/model/fields/reference.py
@@ -2,7 +2,7 @@
 # this repository contains the full copyright notices and license terms.
 import warnings
 
-from sql import Cast, Expression, Literal, Query
+from sql import Cast, Expression, Literal, Null, Query
 from sql.functions import Position, Substring
 
 from trytond.pool import Pool
@@ -214,9 +214,12 @@ class Reference(SelectionMixin, Field):
         if 'active' in Target._fields:
             target_domain.append(('active', 'in', [True, False]))
         query = Target.search(target_domain, order=[], query=True)
-        return (
+        expression = (
             self.sql_id(column, Model).in_(query)
             & column.like(target + ',%'))
+        if operator.startswith('not') or operator == '!=':
+            expression |= column == Null
+        return expression
 
     def definition(self, model, language):
         encoder = PYSONEncoder()
diff --git a/trytond/trytond/tests/test_field_many2one.py b/trytond/trytond/tests/test_field_many2one.py
index 61a258f00f..047a0240a7 100644
--- a/trytond/trytond/tests/test_field_many2one.py
+++ b/trytond/trytond/tests/test_field_many2one.py
@@ -200,6 +200,24 @@ class FieldMany2OneTestCase(unittest.TestCase):
         query = self._test_search_join()
         self.assertNotIsInstance(query.from_[0], Join)
 
+    @with_transaction()
+    def test_search_nested_null(self):
+        "Test search by null many2one"
+        pool = Pool()
+        Many2One = pool.get('test.many2one')
+
+        record, = Many2One.create([{'many2one': None}])
+
+        result = Many2One.search([
+                ('many2one.value', '=', 1),
+                ])
+        self.assertListEqual(result, [])
+
+        result = Many2One.search([
+                ('many2one.value', '!=', 1),
+                ])
+        self.assertListEqual(result, [record])
+
     @with_transaction()
     def test_context_attribute(self):
         "Test context on many2one attribute"
diff --git a/trytond/trytond/tests/test_field_reference.py b/trytond/trytond/tests/test_field_reference.py
index 204af19c07..faf8572257 100644
--- a/trytond/trytond/tests/test_field_reference.py
+++ b/trytond/trytond/tests/test_field_reference.py
@@ -352,6 +352,28 @@ class FieldReferenceTestCase(unittest.TestCase):
 
         self.assertListEqual(references, [reference])
 
+    @with_transaction()
+    def test_search_join_null(self):
+        "Test search by null reference join"
+        pool = Pool()
+        Target = pool.get('test.reference.target')
+        Reference = pool.get('test.reference')
+        reference, = Reference.create([{
+                    'reference': None,
+                    }])
+
+        result = Reference.search([
+                ('reference.name', '=', "Target", Target.__name__),
+                ])
+
+        self.assertListEqual(result, [])
+
+        result = Reference.search([
+                ('reference.name', '!=', "Target", Target.__name__),
+                ])
+
+        self.assertListEqual(result, [reference])
+
     @with_transaction()
     def test_write_string(self):
         "Test write reference string"