diff --git a/pyproject.toml b/pyproject.toml
index 1a57bb7..30dcade 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -45,7 +45,7 @@ classifiers = [
   "Operating System :: MacOS",
 ]
 dependencies = [
-  "numpy>=1.21",
+  "numpy>=1.21, <2",
   "matplotlib>=3.1.1",
   "scipy>=1.3.0",
   "pywavelets>=1.1.1",