diff --git a/runtype/pytypes.py b/runtype/pytypes.py
index 0dfde89..b96a506 100644
--- a/runtype/pytypes.py
+++ b/runtype/pytypes.py
@@ -574,7 +574,7 @@ def _to_canon(self, t):
                 return Tuple
             if Ellipsis in args:
                 if len(args) != 2 or args[0] == Ellipsis:
-                    raise ValueError("Tuple with '...'' expected to be of the exact form: tuple[t, ...].")
+                    raise ValueError("Tuple with '...' expected to be of the exact form: tuple[t, ...].")
                 return TupleEllipsis[to_canon(args[0])]
 
             return ProductType([to_canon(x) for x in args])
@@ -584,7 +584,6 @@ def _to_canon(self, t):
             return SumType(res)
         elif origin is abc.Callable or origin is typing.Callable:
             return Callable[ProductType(to_canon(x) for x in args[:-1]), to_canon(args[-1])]
-            return Callable  # TODO
         elif origin is typing.Literal:
             return OneOf(args)
         elif origin is abc.Mapping or origin is typing.Mapping:
@@ -611,6 +610,8 @@ def _to_canon(self, t):
                 return Type[self.to_canon(t)]
             # TODO test issubclass on t.__args__
             return Type
+        elif isinstance(t, typing._GenericAlias):
+            return self._to_canon(origin)
 
         raise NotImplementedError("No support for type:", t)
 
diff --git a/runtype/validation.py b/runtype/validation.py
index 2a3a56c..9d68599 100644
--- a/runtype/validation.py
+++ b/runtype/validation.py
@@ -1,5 +1,6 @@
 "User-facing API for validation"
 
+from typing import Any, Tuple, Union, Type
 from functools import wraps
 
 from .common import CHECK_TYPES
@@ -23,21 +24,16 @@ def is_subtype(t1, t2):
     return ct1 <= ct2
 
 
-def isa(obj, t):
+def isa(obj: Any, t: Union[Type[Any], Tuple[Type[Any], ...]]) -> bool:
     """Tests if 'obj' is of type 't'
 
     Behaves like Python's isinstance, but supports the ``typing`` module and constraints.
     """
     ct = type_caster.to_canon(t)
     return ct.test_instance(obj)
-    # try:
-    #     ensure_isa(obj, t)
-    #     return True
-    # except TypeMismatchError:
-    #     return False
 
 
-def assert_isa(obj, t):
+def assert_isa(obj: Any, t: Union[Type[Any], Tuple[Type[Any], ...]]):
     """Ensure 'obj' is of type 't'. Otherwise, throws a TypeError
 
     Does nothing if Python is run with -O. (like the assert statement)
@@ -54,7 +50,7 @@ def assert_isa(obj, t):
 
 
 
-def issubclass(t1, t2):
+def issubclass(t1: Type[Any], t2: Union[Type[Any], Tuple[Type[Any], ...]]) -> bool:
     """Test if t1 is a subclass of t2
 
     Parameters:
diff --git a/tests/test_basic.py b/tests/test_basic.py
index 91e2eb2..b737227 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -4,7 +4,7 @@
 import sys
 
 import typing
-from typing import Any, List, Dict, Tuple, Union, Optional, Callable, Set, FrozenSet, Sequence, Type
+from typing import Any, List, Dict, Tuple, Union, Optional, Callable, Set, FrozenSet, Sequence, Type, TypeVar, Generic
 from collections.abc import Iterable
 from dataclasses import FrozenInstanceError, field
 
@@ -717,6 +717,17 @@ class B:
             def a(self, points: list):
                 ...
 
+    def test_generic(self):
+        _Leaf_T = TypeVar("_Leaf_T")
+        class Tree(Generic[_Leaf_T]):
+            pass
+
+        @multidispatch
+        def f(t: Tree[int]):
+            pass
+
+        f(Tree())
+
 
 class TestDataclass(TestCase):
     def setUp(self):