diff --git a/Tests/test_file_fli.py b/Tests/test_file_fli.py
index f087bed00..49c528e11 100644
--- a/Tests/test_file_fli.py
+++ b/Tests/test_file_fli.py
@@ -176,6 +176,7 @@ def test_seek_tell() -> None:
 
 def test_seek() -> None:
     with Image.open(animated_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
         im.seek(50)
 
         assert_image_equal_tofile(im, "Tests/images/a_fli.png")
diff --git a/Tests/test_file_gif.py b/Tests/test_file_gif.py
index 509c74743..f64017118 100644
--- a/Tests/test_file_gif.py
+++ b/Tests/test_file_gif.py
@@ -407,6 +407,7 @@ def test_save_netpbm_l_mode(tmp_path: Path) -> None:
 
 def test_seek() -> None:
     with Image.open("Tests/images/dispose_none.gif") as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         frame_count = 0
         try:
             while True: