diff --git a/src/radical/utils/serialize.py b/src/radical/utils/serialize.py
index d87969ba..61c8072f 100644
--- a/src/radical/utils/serialize.py
+++ b/src/radical/utils/serialize.py
@@ -1,5 +1,4 @@
 
-import copy
 import json
 import msgpack
 
@@ -16,6 +15,7 @@ def __init__(self, ctype, encode, decode):
         self.encode: callable = encode
         self.decode: callable = decode
 
+
 _ctypes = dict()
 
 
@@ -34,14 +34,12 @@ def register_serializable(cls, encode=None, decode=None):
     if encode is None: encode = cls
     if decode is None: decode = cls
 
-    global _ctypes
     _ctypes[cls.__name__] = _CType(cls, encode, decode)
 
 
 # ------------------------------------------------------------------------------
 #
 def _prep_typed_dict(d):
-    from .typeddict import as_dict
     return as_dict(d, _annotate=True)
 
 
@@ -56,13 +54,14 @@ def encode(self, o, *args, **kw):
         tmp = as_dict(o, _annotate=True)
         return super().encode(tmp, *args, **kw)
 
-    def default(self, obj):
-      # print('encode: %s' % obj)
+    def default(self, o):
+      # print('encode: %s' % o)
         for cname,methods in _ctypes.items():
-            if isinstance(obj, methods.ctype):
+            if isinstance(o, methods.ctype):
                 return {'_type': cname,
-                        'as_str': methods.encode(obj)}
-        return super().default(obj)
+                        'as_str': methods.encode(o)}
+        return super().default(o)
+
 
 # ------------------------------------------------------------------------------
 #
diff --git a/src/radical/utils/typeddict.py b/src/radical/utils/typeddict.py
index 5d606827..87ceb6c7 100644
--- a/src/radical/utils/typeddict.py
+++ b/src/radical/utils/typeddict.py
@@ -17,7 +17,6 @@
 #   - optional runtime type checking
 #
 
-import collections
 import copy
 import sys
 
diff --git a/src/radical/utils/zmq/message.py b/src/radical/utils/zmq/message.py
index 01661af1..1fa30bdf 100644
--- a/src/radical/utils/zmq/message.py
+++ b/src/radical/utils/zmq/message.py
@@ -2,7 +2,7 @@
 from typing import Dict, Any
 
 from ..typeddict import TypedDict
-from ..serialize import to_msgpack, from_msgpack, register_serializable
+from ..serialize import to_msgpack, from_msgpack
 
 
 # ------------------------------------------------------------------------------
diff --git a/tests/unittests/test_json.py b/tests/unittests/test_json.py
index a2c0e564..138cd3a4 100644
--- a/tests/unittests/test_json.py
+++ b/tests/unittests/test_json.py
@@ -3,7 +3,7 @@
 # noqa: E201
 
 
-import radical.utils as ru
+# import radical.utils as ru
 
 
 # ------------------------------------------------------------------------------
diff --git a/tests/unittests/test_serialization.py b/tests/unittests/test_serialization.py
index 77fbd9fd..b685171f 100644
--- a/tests/unittests/test_serialization.py
+++ b/tests/unittests/test_serialization.py
@@ -55,8 +55,8 @@ class B(ru.TypedDict):
     new = ru.from_json(ru.to_json(old))
 
     assert old == new
-    assert type(old) == type(new)
-    assert type(old['a']) == type(new['a'])
+    assert isinstance(old, B)      and isinstance(new, B)
+    assert isinstance(old['a'], A) and isinstance(new['a'], A)
 
 
 # ------------------------------------------------------------------------------