diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
index d814dcd..b9ab6bf 100644
--- a/testing/constraints-3.9.txt
+++ b/testing/constraints-3.9.txt
@@ -1,2 +1,3 @@
 # Make sure we test with pandas 1.3.0. The Python version isn't that relevant.
 pandas==1.3.0
+numpy<2.0.0