diff --git a/src/pynwb/image.py b/src/pynwb/image.py
index d8953d397..c399289ed 100644
--- a/src/pynwb/image.py
+++ b/src/pynwb/image.py
@@ -97,7 +97,7 @@ def __init__(self, **kwargs):
             warnings.warn(
                 "%s '%s': The value for 'format' has been changed to 'external'. "
                 "Setting a default value for 'format' is deprecated and will be changed "
-                "to raising a ValueError in the next release."
+                "to raising a ValueError in the next major release."
                 % (self.__class__.__name__, self.name),
                 DeprecationWarning,
             )
diff --git a/tests/back_compat/1.5.1_imageseries_no_data.nwb b/tests/back_compat/1.5.1_imageseries_no_data.nwb
index 298ce5b9c..035614463 100644
Binary files a/tests/back_compat/1.5.1_imageseries_no_data.nwb and b/tests/back_compat/1.5.1_imageseries_no_data.nwb differ
diff --git a/tests/back_compat/1.5.1_imageseries_no_unit.nwb b/tests/back_compat/1.5.1_imageseries_no_unit.nwb
index c238e5fdf..10c6e9f4a 100644
Binary files a/tests/back_compat/1.5.1_imageseries_no_unit.nwb and b/tests/back_compat/1.5.1_imageseries_no_unit.nwb differ
diff --git a/tests/unit/test_image.py b/tests/unit/test_image.py
index 7ac426be8..c279101f5 100644
--- a/tests/unit/test_image.py
+++ b/tests/unit/test_image.py
@@ -52,6 +52,7 @@ def test_external_file_no_frame(self):
             name='test_iS',
             unit='unit',
             external_file=['external_file'],
+            format="external",
             timestamps=[1., 2., 3.]
         )
         self.assertListEqual(iS.starting_frame, [0])
@@ -78,6 +79,7 @@ def test_external_file_no_unit(self):
         iS = ImageSeries(
             name='test_iS',
             external_file=['external_file'],
+            format="external",
             timestamps=list()
         )
         self.assertEqual(iS.unit, ImageSeries.DEFAULT_UNIT)
@@ -242,7 +244,7 @@ def test_external_file_default_format(self):
         msg = (
             "ImageSeries 'test_iS': The value for 'format' has been changed to 'external'. "
             "Setting a default value for 'format' is deprecated and will be changed to "
-            "raising a ValueError in the next release."
+            "raising a ValueError in the next major release."
         )
         with self.assertWarnsWith(DeprecationWarning, msg):
             iS = ImageSeries(