diff --git a/pyproject.toml b/pyproject.toml
index 8cc9ac5..ab015f4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -49,6 +49,9 @@ dependencies = [
     "wsproto",
 ]
 
+[tool.pdm.dependencies]
+greenlet = {version = "*", optional = true, markers = "false"}
+
 [tool.pdm]
 distribution = false