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Author: radarhere

src/PIL/ImageOps.py

@@ -680,7 +680,17 @@ def dither_primary(image: Image.Image) -> Image.Image:
         px = band.load()
         for x in range(0, band.width - 1, 2):
             for y in range(0, band.height - 1, 2):
-                value = (px[x, y] + px[x, y + 1] + px[x + 1, y] + px[x + 1, y + 1]) / 4
+                v1 = px[x, y]
+                v2 = px[x, y + 1]
+                v3 = px[x + 1, y]
+                v4 = px[x + 1, y + 1]
+
+                assert isinstance(v1, (int, float))
+                assert isinstance(v2, (int, float))
+                assert isinstance(v3, (int, float))
+                assert isinstance(v4, (int, float))
+
+                value = (v1 + v2 + v3 + v4) / 4
 
                 px[x, y] = _dither_saturation(value, 0)
                 px[x, y + 1] = _dither_saturation(value, 1)