diff --git a/mongoengine/base.py b/mongoengine/base.py
index 806d83bb..086c7874 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -339,8 +339,8 @@ class BaseDocument(object):
                 try:
                     field._validate(value)
                 except (ValueError, AttributeError, AssertionError), e:
-                    raise ValidationError('Invalid value for field of type "' +
-                                          field.__class__.__name__ + '"')
+                    raise ValidationError('Invalid value for field of type "%s": %s'
+                                          % (field.__class__.__name__, value))
             elif field.required:
                 raise ValidationError('Field "%s" is required' % field.name)
 
diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index 759697ac..f84f751b 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -591,15 +591,20 @@ class FileField(BaseField):
 
     def to_mongo(self, value):
         # Store the GridFS file id in MongoDB
-        return self.gridfs.grid_id
+        if self.gridfs.grid_id is not None:
+            return self.gridfs.grid_id
+        return None
 
     def to_python(self, value):
         # Use stored value (id) to lookup file in GridFS
-        return self.gridfs.get(id=value)
+        if self.gridfs.grid_id is not None:
+            return self.gridfs.get(id=value)
+        return None
 
     def validate(self, value):
-        assert isinstance(value, GridFSProxy)
-        assert isinstance(value.grid_id, pymongo.objectid.ObjectId)
+        if value.grid_id is not None:
+            assert isinstance(value, GridFSProxy)
+            assert isinstance(value.grid_id, pymongo.objectid.ObjectId)
 
 class GeoPointField(BaseField):
     """A list storing a latitude and longitude.