diff --git a/Makefile b/Makefile
index 13d6097..78d20ce 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,7 @@ flake8:
 	flake8 *.py
 
 mypy:
-	mypy --strict --disallow-any-explicit node.py avl.py wavl.py ravl.py
+	mypy --strict --disallow-any-explicit node.py ranked_tree.py avl.py wavl.py ravl.py rbt.py
 
 clean:
 	rm -rf __pycache__ .hypothesis .mypy_cache .pytest_cache
diff --git a/avl.py b/avl.py
index 6b798ac..6b20141 100644
--- a/avl.py
+++ b/avl.py
@@ -114,7 +114,7 @@ class AVLTree(RankedTree[T]):
         return factor != 0
 
     def __delete_fixup(
-        self, x: Optional[Node[T]], parent: Optional[Node[T]] = None
+        self, x: Node[T], parent: Optional[Node[T]] = None
     ) -> bool:
         factor = _balance_factor(x)
         if factor == 0:
@@ -143,7 +143,7 @@ class AVLTree(RankedTree[T]):
         if not node and not parent:
             return
 
-        if not node:
+        if not node and parent:
             node, parent = parent, parent.parent
 
         while node and self.__delete_fixup(node, parent):
diff --git a/node.py b/node.py
index 7494858..584951b 100644
--- a/node.py
+++ b/node.py
@@ -2,6 +2,7 @@ from abc import abstractmethod
 import enum
 from typing import (
     Iterable,
+    Iterator,
     Optional,
     Generic,
     Tuple,
@@ -27,7 +28,7 @@ class NodeType(enum.IntEnum):
 T = TypeVar("T", bound=Comparable)
 
 
-class Node(Generic[T]):
+class Node(Generic[T], Iterable[T]):
     def __init__(
         self,
         value: T,
@@ -61,7 +62,7 @@ class Node(Generic[T]):
     def __str__(self) -> str:
         return f"Node(value={self.value}, rank={self.rank})"
 
-    def __iter__(self) -> Iterable[T]:
+    def __iter__(self) -> Iterator[T]:
         """
         Yields:
             Keys from the subtree rooted at the node in an inorder fashion.
@@ -113,7 +114,7 @@ class Node(Generic[T]):
     @staticmethod
     def find_parent_node(
         value: T, node: "Node[T]", missing: bool = True
-    ) -> "Node[T]":
+    ) -> "Optional[Node[T]]":
         new_node: "Optional[Node[T]]" = node
 
         while new_node and (missing or new_node.value != value):
diff --git a/ranked_tree.py b/ranked_tree.py
index 6b8ce7a..a3b6547 100644
--- a/ranked_tree.py
+++ b/ranked_tree.py
@@ -174,8 +174,7 @@ class RankedTree(Generic[T]):
             self._transplant(node, node.left)
         else:
             n = Node.minimum(node.right)
-            node.value = n.value
-            n.value = None
+            node.value, n.value = n.value, node.value
             return self._delete_node(n)
 
         return (y, parent)
diff --git a/rbt.py b/rbt.py
index 0d24ab9..e7b0819 100644
--- a/rbt.py
+++ b/rbt.py
@@ -18,7 +18,7 @@ class Colour(enum.IntEnum):
     Black = 1
 
 
-def has_double_black(x: Optional[Node]) -> bool:
+def has_double_black(x: Optional[Node[T]]) -> bool:
     """
     Checks for double black child of x.
 
@@ -64,8 +64,8 @@ class RBTree(RankedTree[T]):
         z: Node[T],
         y: Optional[Node[T]],
         right_child: Node[T],
-        rotate_left: RotateFunction,
-        rotate_right: RotateFunction,
+        rotate_left: RotateFunction[T],
+        rotate_right: RotateFunction[T],
     ) -> Node[T]:
         p = z.parent
         pp = p.parent
@@ -114,9 +114,9 @@ class RBTree(RankedTree[T]):
         x: Node[T],
         w: Node[T],
         parent: Node[T],
-        right: Callable[[Node[T]], Node[T]],
-        rotate_left: RotateFunction,
-        rotate_right: RotateFunction,
+        right: Callable[[Node[T]], Optional[Node[T]]],
+        rotate_left: RotateFunction[T],
+        rotate_right: RotateFunction[T],
     ) -> Tuple[Optional[Node[T]], Optional[Node[T]]]:
         if Node.difference(w) == Colour.Red:
             # Case 1