diff --git a/Tests/test_file_tiff.py b/Tests/test_file_tiff.py
index fc16470e1b3..d12a30c5bb7 100644
--- a/Tests/test_file_tiff.py
+++ b/Tests/test_file_tiff.py
@@ -129,11 +129,13 @@ def test_bigtiff_save(self, tmp_path: Path) -> None:
         im.save(outfile, big_tiff=True)
 
         with Image.open(outfile) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2._bigtiff is True
 
         im.save(outfile, save_all=True, append_images=[im], big_tiff=True)
 
         with Image.open(outfile) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2._bigtiff is True
 
     def test_seek_too_large(self) -> None: