diff --git a/pyproject.toml b/pyproject.toml
index a9663f7..300f698 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -44,6 +44,7 @@ docs = [
   "sphinx<7.0.0",
   "sphinx_rtd_theme",
   "m2r2",
+  "setuptools"  # required for sphinx to load m2r2 as an extension
 ]
 dev = [
   "black>=22.10.0",