diff --git a/midas/checker/types.py b/midas/checker/types.py
index 5cb1beb..c16db79 100644
--- a/midas/checker/types.py
+++ b/midas/checker/types.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from typing import Optional
+from typing import Optional, assert_never
 
 
 @dataclass(frozen=True, kw_only=True)
@@ -203,9 +203,13 @@ def substitute_typevars(type: Type, substitutions: dict[str, Type]) -> Type:
         case UnknownType() | UnitType():
             return type
 
-        case _:
+        case TopType() | GenericType():
             raise NotImplementedError(f"Unsupported type {type}")
 
+        # Ensure exhaustiveness
+        case _:
+            assert_never(type)
+
 
 def unfold_type(type: Type) -> Type:
     match type:
diff --git a/midas/generator/generator.py b/midas/generator/generator.py
index 7575ca5..c7b1323 100644
--- a/midas/generator/generator.py
+++ b/midas/generator/generator.py
@@ -2,7 +2,7 @@ import ast
 import shutil
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Optional
+from typing import Optional, assert_never
 
 import midas.ast.python as p
 from midas.ast.location import Location
@@ -19,6 +19,7 @@ from midas.checker.types import (
     Type,
     TypeVar,
     UnitType,
+    UnknownType,
 )
 from midas.utils import TypedAST
 
@@ -276,6 +277,9 @@ class Generator(p.Stmt.Visitor[ast.stmt], p.Expr.Visitor[ast.expr]):
 
     def _make_cast_asserts(self, src_location: Location, expr: ast.expr, type: Type):
         match type:
+            case UnknownType():
+                pass
+
             case BaseType(name=name):
                 self._add_assert(
                     ast.Call(
@@ -301,8 +305,11 @@ class Generator(p.Stmt.Visitor[ast.stmt], p.Expr.Visitor[ast.expr]):
                     self._make_cast_assert_message(src_location, expr, type),
                 )
 
-            case AppliedType():
-                self._make_cast_asserts(src_location, expr, type.body)
+            case AppliedType(body=body):
+                self._make_cast_asserts(src_location, expr, body)
+
+            case TypeVar():
+                raise RuntimeError("Unexpected TypeVar")
 
             case (
                 TopType()
@@ -314,8 +321,9 @@ class Generator(p.Stmt.Visitor[ast.stmt], p.Expr.Visitor[ast.expr]):
             ):
                 raise NotImplementedError(f"Can't make assertion for type {type}")
 
-            case TypeVar():
-                raise RuntimeError("Unexpected TypeVar")
+            # Ensure exhaustiveness
+            case _:
+                assert_never(type)
 
     def _make_cast_assert_message(
         self, location: Location, expr: ast.expr, type: Type