diff --git a/ChangeLog b/ChangeLog
index addd7073..00d13d83 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,7 @@
   for consistency
 - python vips8 binding
 - python vips8 test suite: test_arithmetic.py
+- better extra band handling by colour, again
 
 8/9/14 started 7.40.9
 - support jfif resunit "none"
diff --git a/libvips/colour/colourspace.c b/libvips/colour/colourspace.c
index b033119e..6e514b1f 100644
--- a/libvips/colour/colourspace.c
+++ b/libvips/colour/colourspace.c
@@ -80,7 +80,7 @@ vips_process_n( const char *domain, VipsImage *in, VipsImage **out,
 	if( in->Bands > n ) {
 		VipsImage *scope = vips_image_new();
 		VipsImage **t = (VipsImage **) 
-			vips_object_local_array( VIPS_OBJECT( scope ), 3 );
+			vips_object_local_array( VIPS_OBJECT( scope ), 4 );
 
 		if( vips_extract_band( in, &t[0], 0,
 			"n", n, 
@@ -89,7 +89,8 @@ vips_process_n( const char *domain, VipsImage *in, VipsImage **out,
 				"n", in->Bands - n, 
 				NULL ) ||
 			fn( t[0], &t[2], NULL ) ||
-			vips_bandjoin2( t[2], t[1], out, NULL ) ) {
+			vips_cast( t[1], &t[3], t[2]->BandFmt, NULL ) ||
+			vips_bandjoin2( t[2], t[3], out, NULL ) ) {
 			g_object_unref( scope );
 			return( -1 );
 		}
diff --git a/python/test_colour.py b/python/test_colour.py
index 45a46664..c31d1546 100755
--- a/python/test_colour.py
+++ b/python/test_colour.py
@@ -152,37 +152,42 @@ class TestColour(unittest.TestCase):
     # http://www.brucelindbloom.com
 
     def test_dE00(self):
-        reference = Vips.Image.black(100, 100) + [50, 10, 20]
+        # put 42 in the extra band, it should be copied unmodified
+        reference = Vips.Image.black(100, 100) + [50, 10, 20, 42]
         reference = reference.copy(interpretation = Vips.Interpretation.LAB)
         sample = Vips.Image.black(100, 100) + [40, -20, 10]
         sample = sample.copy(interpretation = Vips.Interpretation.LAB)
 
         difference = reference.dE00(sample)
-        result = difference.getpoint(10, 10)
-        self.assertAlmostEqualObjects(result, [30.238], places = 3)
+        result, alpha = difference.getpoint(10, 10)
+        self.assertAlmostEqual(result, 30.238, places = 3)
+        self.assertAlmostEqual(alpha, 42.0, places = 3)
 
     def test_dE76(self):
-        reference = Vips.Image.black(100, 100) + [50, 10, 20]
+        # put 42 in the extra band, it should be copied unmodified
+        reference = Vips.Image.black(100, 100) + [50, 10, 20, 42]
         reference = reference.copy(interpretation = Vips.Interpretation.LAB)
         sample = Vips.Image.black(100, 100) + [40, -20, 10]
         sample = sample.copy(interpretation = Vips.Interpretation.LAB)
 
         difference = reference.dE76(sample)
-        result = difference.getpoint(10, 10)
-        self.assertAlmostEqualObjects(result, [33.166], places = 3)
+        result, alpha = difference.getpoint(10, 10)
+        self.assertAlmostEqual(result, 33.166, places = 3)
+        self.assertAlmostEqual(alpha, 42.0, places = 3)
 
     # the vips CMC calculation is based on distance in a colorspace derived from
     # the CMC formula, so it won't match exactly ... see vips_LCh2CMC() for
     # details
     def test_dECMC(self):
-        reference = Vips.Image.black(100, 100) + [50, 10, 20]
+        reference = Vips.Image.black(100, 100) + [50, 10, 20, 42]
         reference = reference.copy(interpretation = Vips.Interpretation.LAB)
         sample = Vips.Image.black(100, 100) + [55, 11, 23]
         sample = sample.copy(interpretation = Vips.Interpretation.LAB)
 
         difference = reference.dECMC(sample)
-        [result] = difference.getpoint(10, 10)
+        result, alpha = difference.getpoint(10, 10)
         self.assertLess(abs(result - 4.97), 0.5)
+        self.assertAlmostEqual(alpha, 42.0, places = 3)
 
     # hard to test ICC stuff without including test images 
     # rely on the nip2 test suite for this