diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index 54ce95af7b..ae79764343 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -164,6 +164,9 @@ This document explains the changes made to Iris for this release
 #. `@bjlittle`_ enabled the `sort-all`_ `pre-commit`_ hook to automatically
    sort ``__all__`` entries into alphabetical order. (:pull:`4353`)
 
+#. `@rcomer`_ modified a NetCDF saver test to prevent it triggering a numpy
+   deprecation warning.  (:issue:`4374`, :pull:`4376`)
+
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,
     core dev names are automatically included by the common_links.inc:
diff --git a/lib/iris/tests/unit/fileformats/netcdf/test_Saver.py b/lib/iris/tests/unit/fileformats/netcdf/test_Saver.py
index 656673a7a0..cb5cd2954a 100644
--- a/lib/iris/tests/unit/fileformats/netcdf/test_Saver.py
+++ b/lib/iris/tests/unit/fileformats/netcdf/test_Saver.py
@@ -9,6 +9,7 @@
 # importing anything else.
 import iris.tests as tests  # isort:skip
 
+import collections
 from contextlib import contextmanager
 from unittest import mock
 
@@ -186,6 +187,8 @@ def test_big_endian(self):
     def test_zlib(self):
         cube = self._simple_cube(">f4")
         api = self.patch("iris.fileformats.netcdf.netCDF4")
+        # Define mocked default fill values to prevent deprecation warning (#4374).
+        api.default_fillvals = collections.defaultdict(lambda: -99.0)
         with Saver("/dummy/path", "NETCDF4") as saver:
             saver.write(cube, zlib=True)
         dataset = api.Dataset.return_value