diff --git a/grizzly/adapter/adapter.py b/grizzly/adapter/adapter.py
index 67cc54a6..d0669f04 100644
--- a/grizzly/adapter/adapter.py
+++ b/grizzly/adapter/adapter.py
@@ -3,7 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 from abc import ABCMeta, abstractmethod
 from pathlib import Path
-from typing import Any, Dict, Generator, Optional, Tuple
+from typing import Any, Dict, Generator, Optional, Tuple, final
 
 from grizzly.common.storage import TestCase
 from grizzly.common.utils import DEFAULT_TIME_LIMIT, HARNESS_FILE
@@ -65,6 +65,7 @@ def __enter__(self) -> "Adapter":
     def __exit__(self, *exc: Any) -> None:
         self.cleanup()
 
+    @final
     def cleanup(self) -> None:
         """Automatically called once at shutdown. Used internally by Grizzly.
         *** DO NOT OVERRIDE! ***
@@ -77,6 +78,7 @@ def cleanup(self) -> None:
         """
         self.shutdown()
 
+    @final
     def enable_harness(self, path: Path = HARNESS_FILE) -> None:
         """Enable use of a harness during fuzzing. By default no harness is used.
         *** DO NOT OVERRIDE! ***
@@ -90,6 +92,7 @@ def enable_harness(self, path: Path = HARNESS_FILE) -> None:
         self._harness = path.read_bytes()
         assert self._harness, f"empty harness file '{path.resolve()}'"
 
+    @final
     def get_harness(self) -> Optional[bytes]:
         """Get the harness. Used internally by Grizzly.
         *** DO NOT OVERRIDE! ***
diff --git a/grizzly/target/puppet_target.py b/grizzly/target/puppet_target.py
index fecbb1b7..5f4c498e 100644
--- a/grizzly/target/puppet_target.py
+++ b/grizzly/target/puppet_target.py
@@ -205,7 +205,7 @@ def check_result(self, ignored: Set[str]) -> Result:
                 result = Result.IGNORED
                 LOG.debug("log size limit exceeded")
             else:
-                assert self._puppet.reason
+                assert self._puppet.reason is not None
                 # crash or hang (forced SIGABRT) has been detected
                 LOG.debug("result detected (%s)", self._puppet.reason.name)
                 result = Result.FOUND
diff --git a/grizzly/target/target.py b/grizzly/target/target.py
index 2847c018..582743fb 100644
--- a/grizzly/target/target.py
+++ b/grizzly/target/target.py
@@ -7,7 +7,7 @@
 from os import environ
 from pathlib import Path
 from threading import Lock
-from typing import Any, Dict, Optional, Set, Tuple
+from typing import Any, Dict, Optional, Set, Tuple, final
 
 from ..common.report import Report
 from ..common.utils import CertificateBundle, grz_tmp
@@ -141,6 +141,7 @@ def check_result(self, ignored: Set[str]) -> Result:
             Result code.
         """
 
+    @final
     def cleanup(self) -> None:
         """Perform necessary cleanup. DO NOT OVERRIDE.