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diff --git a/PIL/Image.py b/PIL/Image.py
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index cafc5a2..794ee49 100644
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--- a/PIL/Image.py
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+++ b/PIL/Image.py
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@@ -718,7 +718,7 @@ class Image:
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         if self.mode == "L" and mode == "RGBA" and "transparency" in self.info:
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             from PIL import ImagePalette
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             self.mode = "P"
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-            bytePalette = bytes([i//3 for i in range(768)])
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+            bytePalette = bytes(bytearray([i//3 for i in range(768)]))
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             self.palette = ImagePalette.raw("RGB", bytePalette)
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             self.palette.dirty = 1
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             self.load()
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diff --git a/_imaging.c b/_imaging.c
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index 2ee7eef..6ec7346 100644
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--- a/_imaging.c
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+++ b/_imaging.c
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@@ -286,6 +286,7 @@ static const char* wrong_mode = "unrecognized image mode";
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 static const char* wrong_raw_mode = "unrecognized raw mode";
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 static const char* outside_image = "image index out of range";
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 static const char* outside_palette = "palette index out of range";
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+static const char* wrong_palette_size = "invalid palette size";
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 static const char* no_palette = "image has no palette";
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 static const char* readonly = "image is readonly";
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 /* static const char* no_content = "image has no content"; */
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@@ -1412,6 +1413,11 @@ _putpalette(ImagingObject* self, PyObject* args)
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     return NULL;
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     }
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+    if ( palettesize * 8 / bits > 256) {
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+    PyErr_SetString(PyExc_ValueError, wrong_palette_size);
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+    return NULL;
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+    }
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+
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     ImagingPaletteDelete(self->image->palette);
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     strcpy(self->image->mode, "P");