diff --git a/requirements.txt b/requirements.txt
index d5d560b..955d819 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,6 @@
 tensorflow>=1.14
-tensorflow-estimator>=2.3.0
-mpmath
 scipy>=0.17
+tensorflow-estimator>=2.3.0
+attrs>=21.2.0
+mpmath
 dm-tree~=0.1.1
diff --git a/setup.py b/setup.py
index 1a847f6..32ef7db 100644
--- a/setup.py
+++ b/setup.py
@@ -23,6 +23,7 @@ setup(
     install_requires=[
         'scipy>=0.17',
         'tensorflow-estimator>=2.3.0',  # for DP versions of estimator.
+        'attrs>=21.2.0',  # for tree_aggregation_query.py.
         'mpmath',  # used in tests only
         'dm-tree~=0.1.1',  # used in tests only
     ],