diff --git a/wagtail/images/blocks.py b/wagtail/images/blocks.py
index dc9378c677..b6fc277be2 100644
--- a/wagtail/images/blocks.py
+++ b/wagtail/images/blocks.py
@@ -138,7 +138,7 @@ class ImageBlock(StructBlock):
 
     def to_python(self, value):
         # For backward compatibility with ImageChooserBlock
-        if isinstance(value, int):
+        if value is None or isinstance(value, int):
             image = self.child_blocks["image"].to_python(value)
             struct_value = {"image": image, "decorative": False, "alt_text": None}
         else:
diff --git a/wagtail/images/tests/test_blocks.py b/wagtail/images/tests/test_blocks.py
index 6878678855..64633649ad 100644
--- a/wagtail/images/tests/test_blocks.py
+++ b/wagtail/images/tests/test_blocks.py
@@ -248,6 +248,13 @@ class TestImageBlock(TestImageChooserBlock):
         self.assertEqual(result.contextual_alt_text, "Sample text")
         self.assertFalse(result.decorative)
 
+    def test_to_python_with_none(self):
+        # Like the test_to_python_with_int case, this can occur when a non-required
+        # ImageChooserBlock has been changed to an ImageBlock
+        block = ImageBlock(required=False)
+        value = block.to_python(None)
+        self.assertIsNone(value)
+
     def test_bulk_to_python_with_empty_list(self):
         block = ImageBlock(required=False)
         result = block.bulk_to_python([])