diff --git a/openff/units/exceptions.py b/openff/units/exceptions.py
index 5b2bfe4..9183e81 100644
--- a/openff/units/exceptions.py
+++ b/openff/units/exceptions.py
@@ -1,3 +1,10 @@
+__all__ = [
+    "MissingOpenMMUnitError",
+    "NoneQuantityError",
+    "NoneUnitError",
+]
+
+
 class MissingOpenMMUnitError(Exception):
     """Raised when a unit cannot be converted to an equivalent OpenMM unit"""
 
diff --git a/openff/units/units.py b/openff/units/units.py
index 332425f..30cabfa 100644
--- a/openff/units/units.py
+++ b/openff/units/units.py
@@ -4,7 +4,7 @@
 
 import uuid
 import warnings
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, TypeVar
 
 import pint
 from openff.utilities import requires_package
@@ -52,7 +52,10 @@ def __reduce__(self):
         return _unpickle_unit, (Unit, self._units)
 
 
-class Quantity(_Quantity):
+_MagnitudeType = TypeVar("_MagnitudeType")
+
+
+class Quantity(_Quantity[_MagnitudeType]):
     """A value with associated units."""
 
     _REGISTRY = DEFAULT_UNIT_REGISTRY