diff --git a/expression/collections/map.py b/expression/collections/map.py
index 2383c9c..6c7171a 100644
--- a/expression/collections/map.py
+++ b/expression/collections/map.py
@@ -19,15 +19,16 @@
 from collections.abc import Callable, ItemsView, Iterable, Iterator, Mapping
 from typing import Any, TypeVar, cast
 
-from expression.core import Option, PipeMixin, SupportsLessThan, curry_flip, pipe
+from expression.core import Option, PipeMixin, curry_flip, pipe
+from expression.core.typing import SupportsLessThanAndHash
 
 from . import maptree, seq
 from .block import Block
 from .maptree import MapTree
 
 
-_Key = TypeVar("_Key", bound=SupportsLessThan)
-_Key_ = TypeVar("_Key_", bound=SupportsLessThan)
+_Key = TypeVar("_Key", bound=SupportsLessThanAndHash)
+_Key_ = TypeVar("_Key_", bound=SupportsLessThanAndHash)
 _Value = TypeVar("_Value")
 _Result = TypeVar("_Result")
 
diff --git a/expression/collections/maptree.py b/expression/collections/maptree.py
index efde6e1..2507171 100644
--- a/expression/collections/maptree.py
+++ b/expression/collections/maptree.py
@@ -23,17 +23,18 @@
 """
 
 import builtins
-from collections.abc import Callable, Iterable, Iterator
+from collections.abc import Callable, Hashable, Iterable, Iterator
 from dataclasses import dataclass
 from typing import Any, Generic, TypeVar
 
-from expression.core import Nothing, Option, Some, SupportsLessThan, failwith, pipe
+from expression.core import Nothing, Option, Some, failwith, pipe
+from expression.core.typing import SupportsLessThanAndHash
 
 from . import block, seq
 from .block import Block
 
 
-Key = TypeVar("Key", bound=SupportsLessThan)
+Key = TypeVar("Key", bound=SupportsLessThanAndHash)
 Value = TypeVar("Value")
 Result = TypeVar("Result")
 
@@ -55,7 +56,7 @@ class MapTreeNode(MapTreeLeaf[Key, Value]):
     height: int
 
 
-empty: MapTree[Any, Any] = Nothing
+empty: MapTree[Hashable, Any] = Nothing
 
 
 def is_empty(m: MapTree[Any, Any]):
@@ -72,7 +73,7 @@ def size_aux(acc: int, m: MapTree[Key, Value]) -> int:
             return acc
 
 
-def size(x: MapTree[Any, Any]):
+def size(x: MapTree[Hashable, Any]):
     return size_aux(0, x)
 
 
diff --git a/expression/core/typing.py b/expression/core/typing.py
index 71c30ec..0d9f607 100644
--- a/expression/core/typing.py
+++ b/expression/core/typing.py
@@ -17,6 +17,11 @@ def __lt__(self, __other: Any) -> bool:
         raise NotImplementedError
 
 
+class SupportsLessThanAndHash(SupportsLessThan, Protocol):
+    def __hash__(self) -> int:
+        return super().__hash__()
+
+
 class SupportsSum(Protocol):
     @abstractmethod
     def __radd__(self, __other: Any) -> Any: