diff --git a/pyproject.toml b/pyproject.toml
index 29bbd9f..1c3edb0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -23,7 +23,7 @@ classifiers = [
 ]
 dependencies = [
     "iso8601 >= 0.1",
-    "lxml>=3.4.1",
+    "lxml>=3.4.1,<5.0.0",
 ]
 dynamic = ["version"]