diff --git a/mypyc/lib-rt/setup.py b/mypyc/lib-rt/setup.py
index d82c367aec31..964b56e4b562 100644
--- a/mypyc/lib-rt/setup.py
+++ b/mypyc/lib-rt/setup.py
@@ -11,6 +11,9 @@
 from distutils import ccompiler, sysconfig
 from typing import Any
 
+# we'll import stuff from the source tree, let's ensure is on the sys path
+sys.path.insert(0, os.path.dirname(os.path.realpath(__file__)))
+
 import build_setup  # noqa: F401
 from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext