diff --git a/Tests/images/hopper_mask.ico b/Tests/images/hopper_mask.ico
new file mode 100644
index 00000000000..e8d66c689fd
Binary files /dev/null and b/Tests/images/hopper_mask.ico differ
diff --git a/Tests/images/hopper_mask.png b/Tests/images/hopper_mask.png
new file mode 100644
index 00000000000..c7bd2f70842
Binary files /dev/null and b/Tests/images/hopper_mask.png differ
diff --git a/Tests/test_file_ico.py b/Tests/test_file_ico.py
index 8060d1b763b..317264db646 100644
--- a/Tests/test_file_ico.py
+++ b/Tests/test_file_ico.py
@@ -18,6 +18,11 @@ def test_sanity():
     assert im.get_format_mimetype() == "image/x-icon"
 
 
+def test_mask():
+    with Image.open("Tests/images/hopper_mask.ico") as im:
+        assert_image_equal_tofile(im, "Tests/images/hopper_mask.png")
+
+
 def test_black_and_white():
     with Image.open("Tests/images/black_and_white.ico") as im:
         assert im.mode == "RGBA"
diff --git a/src/PIL/IcoImagePlugin.py b/src/PIL/IcoImagePlugin.py
index ffb1e873d23..d9ff9b5e731 100644
--- a/src/PIL/IcoImagePlugin.py
+++ b/src/PIL/IcoImagePlugin.py
@@ -235,8 +235,8 @@ def frame(self, idx):
                 # the total mask data is
                 # padded row size * height / bits per char
 
-                and_mask_offset = o + int(im.size[0] * im.size[1] * (bpp / 8.0))
                 total_bytes = int((w * im.size[1]) / 8)
+                and_mask_offset = header["offset"] + header["size"] - total_bytes
 
                 self.buf.seek(and_mask_offset)
                 mask_data = self.buf.read(total_bytes)