diff --git a/setup.py b/setup.py
index 556aef8..ee3079b 100644
--- a/setup.py
+++ b/setup.py
@@ -38,6 +38,7 @@ setup(
     keywords="patroni nagios check",
     python_requires=">=3.6",
     install_requires=[
+        "attrs >= 17, !=21.1",
         "urllib3 >= 1.26.6",
         "nagiosplugin >= 1.3.2",
         "click >= 8.0.1",