diff --git a/PIL/Image.py b/PIL/Image.py

index cafc5a2..794ee49 100644

--- a/PIL/Image.py

+++ b/PIL/Image.py

@@ -718,7 +718,7 @@ class Image:

         if self.mode == "L" and mode == "RGBA" and "transparency" in self.info:

             from PIL import ImagePalette

             self.mode = "P"

-            bytePalette = bytes([i//3 for i in range(768)])

+            bytePalette = bytes(bytearray([i//3 for i in range(768)]))

             self.palette = ImagePalette.raw("RGB", bytePalette)

             self.palette.dirty = 1

             self.load()

diff --git a/_imaging.c b/_imaging.c

index 2ee7eef..6ec7346 100644

--- a/_imaging.c

+++ b/_imaging.c

@@ -286,6 +286,7 @@ static const char* wrong_mode = "unrecognized image mode";

 static const char* wrong_raw_mode = "unrecognized raw mode";

 static const char* outside_image = "image index out of range";

 static const char* outside_palette = "palette index out of range";

+static const char* wrong_palette_size = "invalid palette size";

 static const char* no_palette = "image has no palette";

 static const char* readonly = "image is readonly";

 /* static const char* no_content = "image has no content"; */

@@ -1412,6 +1413,11 @@ _putpalette(ImagingObject* self, PyObject* args)

     return NULL;

     }

+    if ( palettesize * 8 / bits > 256) {

+    PyErr_SetString(PyExc_ValueError, wrong_palette_size);

+    return NULL;

+    }

+

     ImagingPaletteDelete(self->image->palette);

     strcpy(self->image->mode, "P");