Converting from APNG to WebP caused an error about timestamp being a float

Tests/test_file_webp_animated.py

@@ -58,6 +58,36 @@ def test_write_animation_L(tmp_path):
             assert_image_similar(im, orig.convert("RGBA"), 32.9)
 
 
+def test_write_animation_float(tmp_path):
+    """
+    Convert an animated PNG to animated WebP, then compare the frame count, and first
+    and last frames to ensure they're visually similar.
+    """
+
+    with Image.open("Tests/images/signalstickercat.png") as orig:
+        assert orig.n_frames > 1
+
+        temp_file = str(tmp_path / "temp.webp")
+        orig.save(temp_file, save_all=True)
+        with Image.open(temp_file) as im:
+            assert im.n_frames == orig.n_frames
+
+            # Compare first and last frames to the original animated GIF
+            orig.load()
+            im.load()
+            assert_image_similar(im, orig.convert("RGBA"), 32.9)
+
+            if is_big_endian():
+                webp = parse_version(features.version_module("webp"))
+                if webp < parse_version("1.2.2"):
+                    pytest.skip("Fails with libwebp earlier than 1.2.2")
+            orig.seek(orig.n_frames - 1)
+            im.seek(im.n_frames - 1)
+            orig.load()
+            im.load()
+            assert_image_similar(im, orig.convert("RGBA"), 32.9)
+
+
 def test_write_animation_RGB(tmp_path):
     """
     Write an animated WebP from RGB frames, and ensure the frames

src/PIL/WebPImagePlugin.py

@@ -285,7 +285,7 @@ def _save_all(im, fp, filename):
                 # Append the frame to the animation encoder
                 enc.add(
                     frame.tobytes("raw", rawmode),
-                    timestamp,
+                    int(timestamp),
                     frame.size[0],
                     frame.size[1],
                     rawmode,
@@ -305,7 +305,7 @@ def _save_all(im, fp, filename):
         im.seek(cur_idx)
 
     # Force encoder to flush frames
-    enc.add(None, timestamp, 0, 0, "", lossless, quality, 0)
+    enc.add(None, int(timestamp), 0, 0, "", lossless, quality, 0)
 
     # Get the final output from the encoder
     data = enc.assemble(icc_profile, exif, xmp)