diff --git a/src/oaklib/io/streaming_writer.py b/src/oaklib/io/streaming_writer.py
index e88878a9d..480bb48d0 100644
--- a/src/oaklib/io/streaming_writer.py
+++ b/src/oaklib/io/streaming_writer.py
@@ -20,7 +20,7 @@
 
 
 @dataclass
-class StreamingWriter(ABC):
+class StreamingWriter:
     """
     Base class for streaming writers.
     """
diff --git a/src/oaklib/utilities/publication_utils/pubdb_wrapper.py b/src/oaklib/utilities/publication_utils/pubdb_wrapper.py
index 916b23175..2c4fb3755 100644
--- a/src/oaklib/utilities/publication_utils/pubdb_wrapper.py
+++ b/src/oaklib/utilities/publication_utils/pubdb_wrapper.py
@@ -1,4 +1,4 @@
-from abc import ABC
+from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from typing import ClassVar, Dict, List, Optional
 
@@ -32,5 +32,6 @@ def set_cache(self, name: str) -> None:
         self.session = requests_cache.CachedSession(name)
         self._uses_cache = True
 
+    @abstractmethod
     def objects_by_ids(self, object_ids: List[str]) -> List[Dict]:
         raise NotImplementedError