diff --git a/environment.yml b/environment.yml
index d69b0a691..fd30be377 100644
--- a/environment.yml
+++ b/environment.yml
@@ -8,6 +8,7 @@ dependencies:
   - pip
   - black
   - nbsphinx
+  - numpy=1.21
   - ctapipe=0.12
   - gammapy=0.19.0
   - cython
diff --git a/setup.py b/setup.py
index 84a35ed41..f8c4a145e 100644
--- a/setup.py
+++ b/setup.py
@@ -39,6 +39,7 @@
         'ctaplot~=0.5.5',
         'gammapy~=0.19.0',
         'uproot~=4.1',
+        'numpy<1.22.0a0',
         'joblib',
         'pandas',
         'pyirf~=0.6.0',