diff --git a/openhtf/output/callbacks/__init__.py b/openhtf/output/callbacks/__init__.py
index 5463c13ce..5b1ba0155 100644
--- a/openhtf/output/callbacks/__init__.py
+++ b/openhtf/output/callbacks/__init__.py
@@ -133,6 +133,8 @@ def __call__(self, test_rec: test_record.TestRecord) -> None:
       serialized_record = self.serialize_test_record(test_rec)
       if isinstance(serialized_record, str):
         outfile.write(serialized_record.encode())
+      elif isinstance(serialized_record, bytes):
+        outfile.write(serialized_record)
       elif isinstance(serialized_record, Iterable):
         for chunk in serialized_record:
           outfile.write(chunk.encode() if isinstance(chunk, str) else chunk)