diff --git a/.github/workflows/linting.yml b/.github/workflows/linting.yml
index 11337c2..cc339a7 100644
--- a/.github/workflows/linting.yml
+++ b/.github/workflows/linting.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.9]
+        python-version: [3.11]
 
     steps:
     - uses: actions/checkout@v3
diff --git a/.github/workflows/testing.yml b/.github/workflows/testing.yml
index fe87df1..4ef1547 100644
--- a/.github/workflows/testing.yml
+++ b/.github/workflows/testing.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.9]
+        python-version: [3.11]
 
     steps:
     - uses: actions/checkout@v3
diff --git a/README.rst b/README.rst
index 7d77589..692f8b6 100644
--- a/README.rst
+++ b/README.rst
@@ -23,7 +23,7 @@ and informative purposes.
 
 Requirements
 ------------
-``Python 3.9`` or newer is required.
+``Python 3.11`` or newer is required.
 
 
 Installation
diff --git a/examples/multithreading_not_safe.py b/examples/multithreading_not_safe.py
index a895c66..6fd4ec7 100644
--- a/examples/multithreading_not_safe.py
+++ b/examples/multithreading_not_safe.py
@@ -1,4 +1,5 @@
 """Demonstrate the trees are not thread-safe in write contention situation."""
+
 import threading
 import sys
 
diff --git a/examples/multithreading_not_safe_read_write.py b/examples/multithreading_not_safe_read_write.py
index 861cb17..2eaf7e7 100644
--- a/examples/multithreading_not_safe_read_write.py
+++ b/examples/multithreading_not_safe_read_write.py
@@ -1,4 +1,5 @@
 """Demonstrate the trees are not thread-safe in read-write contention situation."""
+
 import threading
 import sys
 
diff --git a/examples/multithreading_performance.py b/examples/multithreading_performance.py
index f59e461..8815ee2 100644
--- a/examples/multithreading_performance.py
+++ b/examples/multithreading_performance.py
@@ -1,4 +1,5 @@
 """Module to measure the performance using multithreading."""
+
 import threading
 import time
 
diff --git a/examples/multithreading_safe.py b/examples/multithreading_safe.py
index 2b7e0ea..52587c0 100644
--- a/examples/multithreading_safe.py
+++ b/examples/multithreading_safe.py
@@ -1,4 +1,5 @@
 """Example to show atomic trees are thread-safe in write contention situation."""
+
 import threading
 import sys
 
diff --git a/examples/multithreading_safe_read_write.py b/examples/multithreading_safe_read_write.py
index b0f8aa2..cc1fdf6 100644
--- a/examples/multithreading_safe_read_write.py
+++ b/examples/multithreading_safe_read_write.py
@@ -1,4 +1,5 @@
 """Example to show atomic trees are thread-safe in read-write contention situation."""
+
 import threading
 import sys
 
diff --git a/forest/binary_trees/red_black_tree.py b/forest/binary_trees/red_black_tree.py
index 257c8c6..243532b 100644
--- a/forest/binary_trees/red_black_tree.py
+++ b/forest/binary_trees/red_black_tree.py
@@ -5,8 +5,7 @@
 """Red-Black Tree."""
 
 import enum
-
-from dataclasses import dataclass
+import dataclasses
 
 from typing import Any, Optional, Union
 
@@ -22,14 +21,14 @@ class Color(enum.Enum):
     BLACK = enum.auto()
 
 
-@dataclass
+@dataclasses.dataclass(frozen=True)
 class Leaf:
     """Definition Red-Black Tree Leaf node whose color is always black."""
 
     color = Color.BLACK
 
 
-@dataclass
+@dataclasses.dataclass
 class Node:
     """Red-Black Tree non-leaf node definition."""
 
@@ -518,14 +517,17 @@ def _delete_fixup(self, fixing_node: Union[Leaf, Node]) -> None:
                 else:
                     # Case 3: the sibling is black and its left child is red.
                     if sibling.right.color == Color.BLACK:  # type: ignore
-                        sibling.left.color = Color.BLACK  # type: ignore
+                        if sibling.left.color is not Color.BLACK:
+                            sibling.left.color = Color.BLACK  # type: ignore
                         sibling.color = Color.RED  # type: ignore
                         self._right_rotate(node_x=sibling)  # type: ignore
 
                     # Case 4: the sibling is black and its right child is red.
                     sibling.color = fixing_node.parent.color  # type: ignore
-                    fixing_node.parent.color = Color.BLACK  # type: ignore
-                    sibling.right.color = Color.BLACK  # type: ignore
+                    if fixing_node.parent.color is not Color.BLACK:  # type: ignore
+                        fixing_node.parent.color = Color.BLACK  # type: ignore
+                    if sibling.right.color is not Color.BLACK:
+                        sibling.right.color = Color.BLACK  # type: ignore
                     self._left_rotate(node_x=fixing_node.parent)  # type: ignore
                     # Once we are here, all the violation has been fixed, so
                     # move to the root to terminate the loop.
@@ -552,19 +554,22 @@ def _delete_fixup(self, fixing_node: Union[Leaf, Node]) -> None:
                 else:
                     # Case 7: the sibling is black and its right child is red.
                     if sibling.left.color == Color.BLACK:  # type: ignore
-                        sibling.right.color = Color.BLACK  # type: ignore
+                        if sibling.right.color is not Color.BLACK:
+                            sibling.right.color = Color.BLACK  # type: ignore
                         sibling.color = Color.RED
                         self._left_rotate(node_x=sibling)  # type: ignore
                     # Case 8: the sibling is black and its left child is red.
                     sibling.color = fixing_node.parent.color  # type: ignore
-                    fixing_node.parent.color = Color.BLACK  # type: ignore
-                    sibling.left.color = Color.BLACK  # type: ignore
+                    if fixing_node.parent.color is not Color.BLACK:  # type: ignore
+                        fixing_node.parent.color = Color.BLACK  # type: ignore
+                    if sibling.left.color is not Color.BLACK:
+                        sibling.left.color = Color.BLACK  # type: ignore
                     self._right_rotate(node_x=fixing_node.parent)  # type: ignore
                     # Once we are here, all the violation has been fixed, so
                     # move to the root to terminate the loop.
                     fixing_node = self.root
-
-        fixing_node.color = Color.BLACK
+        if fixing_node.color is not Color.BLACK:
+            fixing_node.color = Color.BLACK
 
     def _transplant(
         self, deleting_node: Node, replacing_node: Union[Node, Leaf]
diff --git a/pyproject.toml b/pyproject.toml
index b1d5866..f1d89ed 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -6,7 +6,7 @@ authors = [
 ]
 description = "The Forest Project in Python"
 readme = {file = "README.rst", content-type = "text/x-rst"}
-requires-python = ">=3.9"
+requires-python = ">=3.11"
 license = {text = "MIT License"}
 classifiers = [
     "License :: OSI Approved :: MIT License",
diff --git a/tests/test_atomic_trees.py b/tests/test_atomic_trees.py
index 495362b..b15db96 100644
--- a/tests/test_atomic_trees.py
+++ b/tests/test_atomic_trees.py
@@ -1,4 +1,5 @@
 """Unit tests for atomic trees."""
+
 import sys
 import threading