diff --git a/CHANGELOG b/CHANGELOG
index 698c98b578..abe080adbc 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Allow PYSON size of fields to be None
 * Add command line completion with argcomplete
 * Include record name and value in validation error message
 * Add header parameter on export data
diff --git a/trytond/model/fields/field.py b/trytond/model/fields/field.py
index 3ce4cbbdbf..47f2aac974 100644
--- a/trytond/model/fields/field.py
+++ b/trytond/model/fields/field.py
@@ -74,7 +74,7 @@ def size_validate(value):
     if value is not None:
         assert isinstance(value, (int, PYSON)), 'size must be PYSON'
         if hasattr(value, 'types'):
-            assert value.types() == {int}, \
+            assert value.types() <= {int, type(None)}, \
                 'size must return integer'
 
 
diff --git a/trytond/model/modelstorage.py b/trytond/model/modelstorage.py
index 974d03ac2a..ef966c5319 100644
--- a/trytond/model/modelstorage.py
+++ b/trytond/model/modelstorage.py
@@ -1395,7 +1395,7 @@ class ModelStorage(Model):
                         else:
                             field_size = field.size
                         size = len(getattr(record, field_name) or '')
-                        if (size > field_size >= 0):
+                        if field_size is not None and (size > field_size >= 0):
                             error_args = cls.__names__(field_name, record)
                             error_args['size'] = size
                             error_args['max_size'] = field_size
diff --git a/trytond/tests/test_field_char.py b/trytond/tests/test_field_char.py
index 7eec01d2d6..eed0032fb2 100644
--- a/trytond/tests/test_field_char.py
+++ b/trytond/tests/test_field_char.py
@@ -444,6 +444,17 @@ class FieldCharTestCase(unittest.TestCase, CommonTestCaseMixin):
                         'size': 5,
                         }])
 
+    @with_transaction()
+    def test_create_size_pyson_none(self):
+        "Test create char with PYSON size as None"
+        pool = Pool()
+        Char = pool.get('test.char_size_pyson')
+
+        char, = Char.create([{
+                    'char': "foo",
+                    'size': None,
+                    }])
+
     @with_transaction()
     def test_create_invalid_char(self):
         "Test create char with invalid char"
diff --git a/trytond/tests/test_field_one2many.py b/trytond/tests/test_field_one2many.py
index a6d62a89bf..922288b55f 100644
--- a/trytond/tests/test_field_one2many.py
+++ b/trytond/tests/test_field_one2many.py
@@ -521,6 +521,19 @@ class FieldOne2ManyTestCase(unittest.TestCase, CommonTestCaseMixin):
                             ],
                         }])
 
+    @with_transaction()
+    def test_create_size_pyson_none(self):
+        "Test create one2many with PYSON size as None"
+        pool = Pool()
+        One2Many = pool.get('test.one2many_size_pyson')
+
+        one2many, = One2Many.create([{
+                    'limit': None,
+                    'targets': [
+                        ('create', [{}]),
+                        ],
+                    }])
+
     @with_transaction()
     def test_create_filter(self):
         "Test create one2many with filter"