diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index df0ac95..1ffb95b 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -32,6 +32,9 @@ jobs:
           cache-dependency-path: pyproject.toml
       - name: Install SimWeights
         run: |
+          if [ "${{matrix.python-version}}" == "3.9" ]; then
+            python3 -m pip install "numpy<2"
+          fi
           python3 -m pip install flit
           python3 -m flit install --symlink --deps=production --extras=test
       - name: Download Test Data