diff --git a/scrapy/contrib_exp/newitem/fields.py b/scrapy/contrib_exp/newitem/fields.py
index 44dbef5e6..97f7efb54 100644
--- a/scrapy/contrib_exp/newitem/fields.py
+++ b/scrapy/contrib_exp/newitem/fields.py
@@ -9,14 +9,9 @@ class BaseField(object):
         self._default = self.to_python(default) if default is not None else None
 
     def to_python(self, value):
-        """
-        Converts the input value into the expected Python data type.
-        Subclasses should override this.
-        """
-        return value
+        raise NotImplementedError()
 
     def get_default(self):
-        """Returns the default value for the field."""
         return self._default
 
 
diff --git a/scrapy/tests/test_newitem.py b/scrapy/tests/test_newitem.py
index d1fbc8dc5..95cced7a0 100644
--- a/scrapy/tests/test_newitem.py
+++ b/scrapy/tests/test_newitem.py
@@ -125,7 +125,7 @@ class NewItemFieldsTest(unittest.TestCase):
         f = fields.BaseField()
 
         assert f.get_default() is None
-        assert f.to_python(1) == 1
+        self.assertRaises(NotImplementedError, f.to_python, 1)
 
     def test_boolean_field(self):
         class TestItem(Item):