diff --git a/pyproject.toml b/pyproject.toml
index b10774a25..522ec0b61 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,7 +28,7 @@ dependencies = [
     "h5py>=3.6.0",
     "xarray>=0.20.2",
     "PyYAML>=6.0",
-    "numpy>=1.21.2",
+    "numpy>=1.21.2,<2.0.0.",
     "pandas>=1.3.2",
     "ase>=3.19.0",
     "mergedeep",