diff --git a/numcodecs/abc.py b/numcodecs/abc.py
index 5aba8c25..a57c13e2 100644
--- a/numcodecs/abc.py
+++ b/numcodecs/abc.py
@@ -29,14 +29,13 @@
 """
 
 from abc import ABC, abstractmethod
-from typing import Optional
+from typing import ClassVar
 
 
 class Codec(ABC):
     """Codec abstract base class."""
 
-    # override in sub-class
-    codec_id: Optional[str] = None
+    codec_id: ClassVar[str]
     """Codec identifier."""
 
     @abstractmethod
diff --git a/numcodecs/gzip.py b/numcodecs/gzip.py
index 4269bad9..90b6df10 100644
--- a/numcodecs/gzip.py
+++ b/numcodecs/gzip.py
@@ -1,5 +1,6 @@
 import gzip as _gzip
 import io
+from typing import ClassVar, Literal
 
 from .abc import Codec
 from .compat import ensure_bytes, ensure_contiguous_ndarray
@@ -15,12 +16,13 @@ class GZip(Codec):
 
     """
 
-    codec_id = 'gzip'
+    codec_id: ClassVar[Literal['gzip']] = 'gzip'
+    level: int
 
     def __init__(self, level=1):
         self.level = level
 
-    def encode(self, buf):
+    def encode(self, buf) -> bytes:
         # normalise inputs
         buf = ensure_contiguous_ndarray(buf)
 
@@ -31,7 +33,7 @@ def encode(self, buf):
         return compressed.getvalue()
 
     # noinspection PyMethodMayBeStatic
-    def decode(self, buf, out=None):
+    def decode(self, buf, out=None) -> bytes:
         # normalise inputs
         # BytesIO only copies if the data is not of `bytes` type.
         # This allows `bytes` objects to pass through without copying.
diff --git a/numcodecs/tests/test_registry.py b/numcodecs/tests/test_registry.py
index 93ed8f03..b5863eb3 100644
--- a/numcodecs/tests/test_registry.py
+++ b/numcodecs/tests/test_registry.py
@@ -34,6 +34,7 @@ def test_all_classes_registered():
         if (
             inspect.isclass(obj)
             and issubclass(obj, numcodecs.abc.Codec)
+            and hasattr(obj, 'codec_id')
             and obj.codec_id not in numcodecs.registry.codec_registry
             and obj.codec_id is not None  # remove `None`
         )