diff --git a/src/infi/clickhouse_orm/query.py b/src/infi/clickhouse_orm/query.py
index 5dfd660..be1b015 100644
--- a/src/infi/clickhouse_orm/query.py
+++ b/src/infi/clickhouse_orm/query.py
@@ -1,4 +1,5 @@
 from __future__ import unicode_literals
+
 import six
 import pytz
 from copy import copy
@@ -408,7 +409,7 @@ class QuerySet(object):
         Adds a FINAL modifier to table, meaning data will be collapsed to final version.
         Can be used with CollapsingMergeTree engine only
         """
-        if not issubclass(self._model_cls, CollapsingMergeTree):
+        if not isinstance(self._model_cls.engine, CollapsingMergeTree):
             raise TypeError('final() method can be used only with CollapsingMergeTree engine')
 
         qs = copy(self)
diff --git a/tests/test_querysets.py b/tests/test_querysets.py
index 0fcc871..6410672 100644
--- a/tests/test_querysets.py
+++ b/tests/test_querysets.py
@@ -419,7 +419,7 @@ class SampleModel(Model):
 
 class SampleCollapsingModel(SampleModel):
 
-    sign = UInt8Field(default=1)
+    sign = Int8Field(default=1)
 
     engine = CollapsingMergeTree('materialized_date', ('num',), 'sign')