diff --git a/Tests/test_lib_pack.py b/Tests/test_lib_pack.py
index 461d0bf8e..83872c5d1 100644
--- a/Tests/test_lib_pack.py
+++ b/Tests/test_lib_pack.py
@@ -10,11 +10,11 @@ class TestLibPack(PillowTestCase):
 
     def test_pack(self):
 
-        def pack(mode, rawmode):
+        def pack(mode, rawmode, in_data=(1, 2, 3, 4)):
             if len(mode) == 1:
-                im = Image.new(mode, (1, 1), 1)
+                im = Image.new(mode, (1, 1), in_data[0])
             else:
-                im = Image.new(mode, (1, 1), (1, 2, 3, 4)[:len(mode)])
+                im = Image.new(mode, (1, 1), in_data[:len(mode)])
 
             if py3:
                 return list(im.tobytes("raw", rawmode))
@@ -47,6 +47,8 @@ class TestLibPack(PillowTestCase):
         self.assertEqual(pack("RGBX", "RGBX"), [1, 2, 3, 4])  # 4->255?
 
         self.assertEqual(pack("RGBA", "RGBA"), [1, 2, 3, 4])
+        self.assertEqual(pack("RGBA", "BGRa"), [0, 0, 0, 4])
+        self.assertEqual(pack("RGBA", "BGRa", in_data=(20, 30, 40, 50)), [8, 6, 4, 50])
 
         self.assertEqual(pack("RGBa", "RGBa"), [1, 2, 3, 4])
         self.assertEqual(pack("RGBa", "BGRa"), [3, 2, 1, 4])
@@ -122,7 +124,9 @@ class TestLibPack(PillowTestCase):
         self.assertEqual(unpack("RGB", "XBGR", 4), (4, 3, 2))
 
         self.assertEqual(unpack("RGBA", "RGBA", 4), (1, 2, 3, 4))
+        self.assertEqual(unpack("RGBA", "RGBa", 4), (63, 127, 191, 4))
         self.assertEqual(unpack("RGBA", "BGRA", 4), (3, 2, 1, 4))
+        self.assertEqual(unpack("RGBA", "BGRa", 4), (191, 127, 63, 4))
         self.assertEqual(unpack("RGBA", "ARGB", 4), (2, 3, 4, 1))
         self.assertEqual(unpack("RGBA", "ABGR", 4), (4, 3, 2, 1))
         self.assertEqual(unpack("RGBA", "RGBA;15", 2), (8, 131, 0, 0))
diff --git a/encode.c b/encode.c
index b08a9aa57..9a0dd2446 100644
--- a/encode.c
+++ b/encode.c
@@ -365,7 +365,7 @@ get_packer(ImagingEncoderObject* encoder, const char* mode,
     pack = ImagingFindPacker(mode, rawmode, &bits);
     if (!pack) {
         Py_DECREF(encoder);
-        PyErr_SetString(PyExc_SystemError, "unknown raw mode");
+        PyErr_Format(PyExc_ValueError, "No packer found from %s to %s", mode, rawmode);
         return -1;
     }
 
diff --git a/libImaging/Pack.c b/libImaging/Pack.c
index d83cb8284..a19252e88 100644
--- a/libImaging/Pack.c
+++ b/libImaging/Pack.c
@@ -72,6 +72,10 @@
 #define C64L C64N
 #endif
 
+/* like (a * b + 127) / 255), but much faster on most platforms */
+#define MULDIV255(a, b, tmp)\
+        (tmp = (a) * (b) + 128, ((((tmp) >> 8) + (tmp)) >> 8))
+
 static void
 pack1(UINT8* out, const UINT8* in, int pixels)
 {
@@ -315,6 +319,21 @@ ImagingPackABGR(UINT8* out, const UINT8* in, int pixels)
     }
 }
 
+void
+ImagingPackBGRa(UINT8* out, const UINT8* in, int pixels)
+{
+    int i;
+    /* BGRa, reversed bytes with premultiplied alplha */
+    for (i = 0; i < pixels; i++) {
+        int alpha = out[3] = in[A];
+        int tmp;
+        out[0] = MULDIV255(in[B], alpha, tmp);
+        out[1] = MULDIV255(in[G], alpha, tmp);
+        out[2] = MULDIV255(in[R], alpha, tmp);
+        out += 4; in += 4;
+    }
+}
+
 static void
 packRGBL(UINT8* out, const UINT8* in, int pixels)
 {
@@ -525,6 +544,7 @@ static struct {
     {"RGBA",	"BGR",		24,	ImagingPackBGR},
     {"RGBA",	"BGRA",		32,	ImagingPackBGRA},
     {"RGBA",	"ABGR",		32,	ImagingPackABGR},
+    {"RGBA",	"BGRa",		32,	ImagingPackBGRa},
     {"RGBA",   	"R",            8,      band0},
     {"RGBA",   	"G",            8,      band1},
     {"RGBA",   	"B",            8,      band2},
diff --git a/libImaging/Unpack.c b/libImaging/Unpack.c
index 702bc9f1f..8112afd8d 100644
--- a/libImaging/Unpack.c
+++ b/libImaging/Unpack.c
@@ -747,6 +747,30 @@ unpackRGBa(UINT8* out, const UINT8* in, int pixels)
     }
 }
 
+static void
+unpackBGRa(UINT8* out, const UINT8* in, int pixels)
+{
+    int i;
+    /* premultiplied BGRA */
+    for (i = 0; i < pixels; i++) {
+        int a = in[3];
+        if (!a)
+            out[R] = out[G] = out[B] = out[A] = 0;
+        else if (a == 255) {
+            out[R] = in[2];
+            out[G] = in[1];
+            out[B] = in[0];
+            out[A] = a;
+        } else {
+            out[R] = CLIP(in[2] * 255 / a);
+            out[G] = CLIP(in[1] * 255 / a);
+            out[B] = CLIP(in[0] * 255 / a);
+            out[A] = a;
+        }
+        out += 4; in += 4;
+    }
+}
+
 static void
 unpackRGBAI(UINT8* out, const UINT8* in, int pixels)
 {
@@ -1206,6 +1230,7 @@ static struct {
     {"RGBA",    "LA;16B",       32,     unpackRGBALA16B},
     {"RGBA",    "RGBA",         32,     copy4},
     {"RGBA",    "RGBa",         32,     unpackRGBa},
+    {"RGBA",    "BGRa",         32,     unpackBGRa},
     {"RGBA",    "RGBA;I",       32,     unpackRGBAI},
     {"RGBA",    "RGBA;L",       32,     unpackRGBAL},
     {"RGBA",    "RGBA;15",      16,     ImagingUnpackRGBA15},