diff --git a/setup.cfg b/setup.cfg
index 6fae27d..1fae380 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -34,7 +34,8 @@ classifiers =
 [options]
 python_requires = >=3.6
 install_requires =
-    attrs
+    # >= 20.1.0 for attr.frozen
+    attrs >= 20.1.0
     typing-extensions ~= 3.10
 include_package_data = True
 zip_safe = False