diff --git a/pyproject.toml b/pyproject.toml
index b8eaa23..380d95b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,5 +4,6 @@ requires = [
     "numpy>=2; python_version>='3.9'",
     "oldest-supported-numpy; python_version<'3.9'",
     "cython",
+    "cython<3.1.0; python_version<'3.9'",
 ]
 build-backend = "setuptools.build_meta"