diff --git a/pyproject.toml b/pyproject.toml
index 0ebc928..b159ecb 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -21,7 +21,7 @@ dependencies = [
     "f90nml",
     "mule",
     "netcdf4",
-    "numpy",
+    "numpy <2",
     "scitools-iris",
     "versioneer",
 ]
@@ -51,4 +51,4 @@ style = "pep440"
 versionfile_source = "umpost/_version.py"
 versionfile_build = "umpost/_version.py"
 tag_prefix = ""
-parentdir_prefix = "umpost-"
\ No newline at end of file
+parentdir_prefix = "umpost-"