Check quality_layers type

Tests/test_file_jpeg2k.py

@@ -105,6 +105,22 @@ class TestFileJpeg2k(PillowTestCase):
         im.load()
         self.assertEqual(im.size, (160, 120))
 
+    def test_layers_type(self):
+        outfile = self.tempfile('temp_layers.jp2')
+        for quality_layers in [
+            [100, 50, 10],
+            (100, 50, 10),
+            None
+        ]:
+            test_card.save(outfile, quality_layers=quality_layers)
+
+        for quality_layers in [
+            'quality_layers',
+            ('100', '50', '10')
+        ]:
+            self.assertRaises(ValueError, test_card.save, outfile,
+                              quality_layers=quality_layers)
+
     def test_layers(self):
         out = BytesIO()
         test_card.save(out, 'JPEG2000', quality_layers=[100, 50, 10],

src/PIL/Jpeg2KImagePlugin.py

@@ -232,6 +232,13 @@ def _save(im, fp, filename):
     tile_size = info.get('tile_size', None)
     quality_mode = info.get('quality_mode', 'rates')
     quality_layers = info.get('quality_layers', None)
+    if quality_layers is not None and not (
+        isinstance(quality_layers, (list, tuple)) and
+        all([isinstance(quality_layer, (int, float))
+             for quality_layer in quality_layers])
+    ):
+        raise ValueError('quality_layers must be a sequence of numbers')
+
     num_resolutions = info.get('num_resolutions', 0)
     cblk_size = info.get('codeblock_size', None)
     precinct_size = info.get('precinct_size', None)