1 files changed, 9 insertions, 10 deletions
diff --git a/pint/facets/system/registry.py b/pint/facets/system/registry.py
index 30921bd..04aaea7 100644
--- a/pint/facets/system/registry.py
+++ b/pint/facets/system/registry.py
@@ -9,7 +9,7 @@
 from __future__ import annotations
 
 from numbers import Number
-from typing import TYPE_CHECKING, Generic, Any
+from typing import TYPE_CHECKING, Generic, Any, Union, Optional
 
 from ... import errors
 
@@ -53,17 +53,16 @@ class GenericSystemRegistry(
     # to enjoy typing goodies
     System: type[objects.System]
 
-    def __init__(self, system: str | None = None, **kwargs):
+    def __init__(self, system: Optional[str] = None, **kwargs):
         super().__init__(**kwargs)
 
         #: Map system name to system.
-        #: :type: dict[ str | System]
         self._systems: dict[str, objects.System] = {}
 
         #: Maps dimensionality (UnitsContainer) to Dimensionality (UnitsContainer)
         self._base_units_cache: dict[UnitsContainerT, UnitsContainerT] = {}
 
-        self._default_system_name: str | None = system
+        self._default_system_name: Optional[str] = system
 
     def _init_dynamic_classes(self) -> None:
         """Generate subclasses on the fly and attach them to self"""
@@ -104,7 +103,7 @@ class GenericSystemRegistry(
         return objects.Lister(self._systems)
 
     @property
-    def default_system(self) -> str | None:
+    def default_system(self) -> Optional[str]:
         return self._default_system_name
 
     @default_system.setter
@@ -144,9 +143,9 @@ class GenericSystemRegistry(
 
     def get_base_units(
         self,
-        input_units: UnitLike | Quantity,
+        input_units: Union[UnitLike, Quantity],
         check_nonmult: bool = True,
-        system: str | objects.System | None = None,
+        system: Optional[Union[str, objects.System]] = None,
     ) -> tuple[Number, Unit]:
         """Convert unit or dict of units to the plain units.
 
@@ -184,7 +183,7 @@ class GenericSystemRegistry(
         self,
         input_units: UnitsContainerT,
         check_nonmult: bool = True,
-        system: str | objects.System | None = None,
+        system: Optional[Union[str, objects.System]] = None,
     ):
         if system is None:
             system = self._default_system_name
@@ -226,7 +225,7 @@ class GenericSystemRegistry(
         return base_factor, destination_units
 
     def get_compatible_units(
-        self, input_units: UnitsContainerT, group_or_system: str | None = None
+        self, input_units: UnitsContainerT, group_or_system: Optional[str] = None
     ) -> frozenset[Unit]:
         """ """
 
@@ -242,7 +241,7 @@ class GenericSystemRegistry(
         return frozenset(self.Unit(eq) for eq in equiv)
 
     def _get_compatible_units(
-        self, input_units: UnitsContainerT, group_or_system: str | None = None
+        self, input_units: UnitsContainerT, group_or_system: Optional[str] = None
     ) -> frozenset[Unit]:
         if group_or_system and group_or_system in self._systems:
             members = self._systems[group_or_system].members