diff --git a/swh/model/tests/test_hypothesis_strategies.py b/swh/model/tests/test_hypothesis_strategies.py
index 52e1211..7e63d84 100644
--- a/swh/model/tests/test_hypothesis_strategies.py
+++ b/swh/model/tests/test_hypothesis_strategies.py
@@ -1,38 +1,54 @@
 # Copyright (C) 2019 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import attr
 from hypothesis import given
 
 from swh.model.hashutil import DEFAULT_ALGORITHMS
 from swh.model.hypothesis_strategies import objects, object_dicts
 
 
 target_types = (
     'content', 'directory', 'revision', 'release', 'snapshot', 'alias')
 
 
 @given(objects())
 def test_generation(obj_type_and_obj):
     (obj_type, object_) = obj_type_and_obj
     attr.validate(object_)
 
 
+def assert_nested_dict(obj):
+    """Tests the object is a nested dict and contains no more class
+    from swh.model.model."""
+    if isinstance(obj, dict):
+        for (key, value) in obj.items():
+            assert isinstance(key, (str, bytes)), key
+            assert_nested_dict(value)
+    elif isinstance(obj, list):
+        for value in obj:
+            assert_nested_dict(value)
+    elif isinstance(obj, (int, float, str, bytes, bool, type(None))):
+        pass
+    else:
+        assert False, obj
+
+
 @given(object_dicts())
 def test_dicts_generation(obj_type_and_obj):
     (obj_type, object_) = obj_type_and_obj
-    assert isinstance(object_, dict)
+    assert_nested_dict(object_)
     if obj_type == 'content':
         if object_['status'] == 'visible':
             assert set(object_) == \
                 set(DEFAULT_ALGORITHMS) | {'length', 'status', 'data'}
         else:
             assert set(object_) == \
                 set(DEFAULT_ALGORITHMS) | {'length', 'status'}
     elif obj_type == 'release':
         assert object_['target_type'] in target_types
     elif obj_type == 'snapshot':
         for branch in object_['branches'].values():
             assert branch['target_type'] in target_types