diff --git a/flit_core/pyproject.toml b/flit_core/pyproject.toml
index affb6d0a..a027ff0c 100644
--- a/flit_core/pyproject.toml
+++ b/flit_core/pyproject.toml
@@ -24,4 +24,8 @@ Documentation = "https://flit.pypa.io"
 Source = "https://github.com/pypa/flit"
 
 [tool.flit.sdist]
-include = ["bootstrap_install.py", "build_dists.py"]
+include = [
+    "bootstrap_install.py",
+    "build_dists.py",
+    "tests_core/",
+]