diff --git a/mypyc/build.py b/mypyc/build.py
index cc03eba95b4e..f90e82abc7fc 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -25,7 +25,7 @@
 import re
 import sys
 import time
-from typing import TYPE_CHECKING, Any, Dict, Iterable, NoReturn, cast
+from typing import TYPE_CHECKING, Any, Dict, Iterable, NoReturn, Union, cast
 
 from mypy.build import BuildSource
 from mypy.errors import CompileError
@@ -41,7 +41,13 @@
 from mypyc.options import CompilerOptions
 
 if TYPE_CHECKING:
-    from distutils.core import Extension
+    from distutils.core import Extension as _distutils_Extension
+    from typing_extensions import TypeAlias
+
+    from setuptools import Extension as _setuptools_Extension
+
+    Extension: TypeAlias = Union[_setuptools_Extension, _distutils_Extension]
+
 
 try:
     # Import setuptools so that it monkey-patch overrides distutils
diff --git a/setup.py b/setup.py
index 516a639f3bb2..5d5ea06fb714 100644
--- a/setup.py
+++ b/setup.py
@@ -6,6 +6,7 @@
 import os
 import os.path
 import sys
+from typing import TYPE_CHECKING, Any
 
 if sys.version_info < (3, 7, 0):
     sys.stderr.write("ERROR: You need Python 3.7 or later to use mypy.\n")
@@ -17,11 +18,14 @@
 # This requires setuptools when building; setuptools is not needed
 # when installing from a wheel file (though it is still needed for
 # alternative forms of installing, as suggested by README.md).
-from setuptools import find_packages, setup
+from setuptools import Extension, find_packages, setup
 from setuptools.command.build_py import build_py
 
 from mypy.version import __version__ as version
 
+if TYPE_CHECKING:
+    from typing_extensions import TypeGuard
+
 description = "Optional static typing for Python"
 long_description = """
 Mypy -- Optional Static Typing for Python
@@ -36,6 +40,10 @@
 """.lstrip()
 
 
+def is_list_of_setuptools_extension(items: list[Any]) -> TypeGuard[list[Extension]]:
+    return all(isinstance(item, Extension) for item in items)
+
+
 def find_package_data(base, globs, root="mypy"):
     """Find all interesting data files, for setup(package_data=)
 
@@ -166,6 +174,8 @@ def run(self):
         # our Appveyor builds run out of memory sometimes.
         multi_file=sys.platform == "win32" or force_multifile,
     )
+    assert is_list_of_setuptools_extension(ext_modules), "Expected mypycify to use setuptools"
+
 else:
     ext_modules = []