diff --git a/src/plumpy/base/state_machine.py b/src/plumpy/base/state_machine.py
index 4e7a5722..62062d5d 100644
--- a/src/plumpy/base/state_machine.py
+++ b/src/plumpy/base/state_machine.py
@@ -26,7 +26,7 @@ class StateMachineError(Exception):
     """Base class for state machine errors"""
 
 
-class StateEntryFailed(Exception):
+class StateEntryFailed(Exception):  # noqa: N818
     """
     Failed to enter a state, can provide the next state to go to via this exception
     """
@@ -48,7 +48,7 @@ def __init__(self, evt: str, msg: str):
         self.event = evt
 
 
-class TransitionFailed(Exception):
+class TransitionFailed(Exception):  # noqa: N818
     """A state transition failed"""
 
     def __init__(
diff --git a/src/plumpy/lang.py b/src/plumpy/lang.py
index 9672d500..aad2abb6 100644
--- a/src/plumpy/lang.py
+++ b/src/plumpy/lang.py
@@ -67,7 +67,7 @@ def wrapped_fn(self: Any, *args: Any, **kwargs: Any) -> Callable[..., Any]:
     return wrap
 
 
-class __NULL:  # pylint: disable=invalid-name
+class __NULL:  # noqa: N801
     def __eq__(self, other: Any) -> bool:
         return isinstance(other, self.__class__)
 
diff --git a/src/plumpy/loaders.py b/src/plumpy/loaders.py
index 59f33f64..2da2e57c 100644
--- a/src/plumpy/loaders.py
+++ b/src/plumpy/loaders.py
@@ -74,7 +74,7 @@ def get_object_loader() -> ObjectLoader:
     :return: A class loader
     :rtype: :class:`ObjectLoader`
     """
-    global OBJECT_LOADER
+    global OBJECT_LOADER  # noqa: PLW0603
     if OBJECT_LOADER is None:
         OBJECT_LOADER = DefaultObjectLoader()
     return OBJECT_LOADER
@@ -88,5 +88,5 @@ def set_object_loader(loader: Optional[ObjectLoader]) -> None:
     :type loader: :class:`ObjectLoader`
     :return:
     """
-    global OBJECT_LOADER
+    global OBJECT_LOADER  # noqa: PLW0603
     OBJECT_LOADER = loader
diff --git a/test/test_port.py b/test/test_port.py
index b0535fdb..da483e81 100644
--- a/test/test_port.py
+++ b/test/test_port.py
@@ -80,7 +80,7 @@ def test_lambda_default(self):
         port = InputPort('test', valid_type=(types.FunctionType, int), default=lambda: 5)
 
         def some_lambda():
-            "string"
+            return 'string'
 
         self.assertIsNone(port.validate(some_lambda))