Fix assertion observed when adding new colors to an indexed-color TIFF

Tests/test_tiff_palette.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+import io
+
+from PIL import Image, ImageDraw
+
+
+def test_tiff_palette() -> None:
+    image = Image.new("P", (3, 1))
+    file = io.BytesIO()
+    image.save(file, "tiff")
+
+    image = Image.open(file)
+    draw = ImageDraw.Draw(image)
+    COLOR_0 = (1, 2, 3)
+    COLOR_1 = (127, 128, 129)
+    COLOR_2 = (252, 253, 254)
+    draw.point((0, 0), fill=COLOR_0)
+    draw.point((1, 0), fill=COLOR_1)
+    draw.point((2, 0), fill=COLOR_2)
+
+    converted_data = list(image.convert("RGB").getdata())
+    assert len(converted_data) == 3
+    assert converted_data[0] == COLOR_0
+    assert converted_data[1] == COLOR_1
+    assert converted_data[2] == COLOR_2

src/PIL/ImagePalette.py

@@ -166,7 +166,7 @@ def getcolor(
             except KeyError as e:
                 # allocate new color slot
                 index = self._new_color_index(image, e)
-                assert isinstance(self._palette, bytearray)
+                assert isinstance(self._palette, (bytes, bytearray))
                 self.colors[color] = index
                 if index * 3 < len(self.palette):
                     self._palette = (