diff --git a/setup.py b/setup.py
index bc28b50..3279165 100644
--- a/setup.py
+++ b/setup.py
@@ -7,7 +7,7 @@
 from setuptools.command.build_ext import build_ext as build_ext
 
 
-CFLAGS = ['-O2']
+CFLAGS = ['-O2', '-DCYTHON_FREETHREADING_COMPATIBLE=1']
 
 ROOT = pathlib.Path(__file__).parent