diff --git a/Tests/images/bc4u.dds b/Tests/images/bc4u.dds
new file mode 100644
index 00000000000..7f9f050b6f1
Binary files /dev/null and b/Tests/images/bc4u.dds differ
diff --git a/Tests/images/unimplemented_fourcc.dds b/Tests/images/unimplemented_fourcc.dds
deleted file mode 100755
index 4795573646b..00000000000
Binary files a/Tests/images/unimplemented_fourcc.dds and /dev/null differ
diff --git a/Tests/images/unknown_fourcc.dds b/Tests/images/unknown_fourcc.dds
deleted file mode 100755
index 41a34388615..00000000000
Binary files a/Tests/images/unknown_fourcc.dds and /dev/null differ
diff --git a/Tests/test_file_dds.py b/Tests/test_file_dds.py
index def548a34c5..b33c01a203a 100644
--- a/Tests/test_file_dds.py
+++ b/Tests/test_file_dds.py
@@ -17,6 +17,7 @@
 TEST_FILE_DX10_BC5_SNORM = "Tests/images/bc5_snorm.dds"
 TEST_FILE_DX10_BC1 = "Tests/images/bc1.dds"
 TEST_FILE_DX10_BC1_TYPELESS = "Tests/images/bc1_typeless.dds"
+TEST_FILE_BC4U = "Tests/images/bc4u.dds"
 TEST_FILE_BC5S = "Tests/images/bc5s.dds"
 TEST_FILE_BC5U = "Tests/images/bc5u.dds"
 TEST_FILE_BC6H = "Tests/images/bc6h.dds"
@@ -80,10 +81,18 @@ def test_sanity_dxt5():
     assert_image_equal_tofile(im, TEST_FILE_DXT5.replace(".dds", ".png"))
 
 
-def test_sanity_ati1():
+@pytest.mark.parametrize(
+    "image_path",
+    (
+        TEST_FILE_ATI1,
+        # hexeditted to use BC4U FourCC
+        TEST_FILE_BC4U,
+    ),
+)
+def test_sanity_ati1_bc4u(image_path):
     """Check ATI1 images can be opened"""
 
-    with Image.open(TEST_FILE_ATI1) as im:
+    with Image.open(image_path) as im:
         im.load()
 
         assert im.format == "DDS"
@@ -210,12 +219,6 @@ def test_dx10_r8g8b8a8_unorm_srgb():
         )
 
 
-def test_unimplemented_dxgi_format():
-    with pytest.raises(NotImplementedError):
-        with Image.open("Tests/images/unimplemented_dxgi_format.dds"):
-            pass
-
-
 @pytest.mark.parametrize(
     ("mode", "size", "test_file"),
     [
diff --git a/src/PIL/DdsImagePlugin.py b/src/PIL/DdsImagePlugin.py
index 581e60a3f08..30f021b7dc5 100644
--- a/src/PIL/DdsImagePlugin.py
+++ b/src/PIL/DdsImagePlugin.py
@@ -345,11 +345,11 @@ def _open(self):
 
         # pixel format
         pfsize, pfflags, fourcc, bitcount = struct.unpack("<4I", header.read(16))
-        masks = struct.unpack("<4I", header.read(16))
         n = 0
         rawmode = None
         if pfflags & DDPF.RGB:
             # Texture contains uncompressed RGB data
+            masks = struct.unpack("<4I", header.read(16))
             masks = {mask: ["R", "G", "B", "A"][i] for i, mask in enumerate(masks)}
             if bitcount == 8:
                 self._mode = "L"