Fix rgb2grey for alpha channel

skimage/color/colorconv.py

@@ -711,7 +711,7 @@ def rgb2gray(rgb):
     if rgb.ndim == 2:
         return np.ascontiguousarray(rgb)
 
-    rgb = _prepare_colorarray(rgb)
+    rgb = _prepare_colorarray(rgb[..., :3])
 
     gray = 0.2125 * rgb[..., 0]
     gray[:] += 0.7154 * rgb[..., 1]

skimage/color/tests/test_colorconv.py

@@ -231,6 +231,10 @@ class TestColorconv(TestCase):
         assert rgb2grey(x).flags["C_CONTIGUOUS"]
         assert rgb2grey(x[:5, :5]).flags["C_CONTIGUOUS"]
 
+    def test_rgb2grey_alpha(self):
+        x = np.random.rand(10, 10, 4)
+        assert rgb2grey(x).ndim == 2
+
     def test_rgb2grey_on_grey(self):
         rgb2grey(np.random.rand(5, 5))