From 8f18e22eb41e087d3c8ed380beb29ea39d3e9436 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Fri, 4 Sep 2026 14:25:02 +1000
Subject: [PATCH] Correct error message for matrix length

---
 Tests/test_image_convert.py | 18 ++++++++++++------
 src/_imaging.c              |  1 +
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/Tests/test_image_convert.py b/Tests/test_image_convert.py
index bf5034cc932..e1b06699c8f 100644
--- a/Tests/test_image_convert.py
+++ b/Tests/test_image_convert.py
@@ -46,7 +46,7 @@ def convert(im: Image.Image, mode: str) -> None:
 
 def test_unsupported_conversion() -> None:
     im = hopper()
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match="image has wrong mode"):
         im.convert("INVALID")
 
 
@@ -305,26 +305,33 @@ def test_matrix_illegal_conversion() -> None:
     assert im.mode != "RGB"
 
     # Act / Assert
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match="illegal conversion"):
         im.convert(mode="CMYK", matrix=rgb2xyz_matrix)
 
 
 def test_matrix_wrong_mode() -> None:
     # Arrange
     im = hopper("L")
-    assert im.mode == "L"
 
     # Act / Assert
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match="image has wrong mode"):
         im.convert(mode="L", matrix=rgb2xyz_matrix)
 
 
+def test_matrix_truncated() -> None:
+    # Arrange
+    im = hopper()
+
+    # Act / Assert
+    with pytest.raises(TypeError, match="matrix must be tuple of length 4 or 12"):
+        im.convert(mode="L", matrix=(0,))
+
+
 @pytest.mark.parametrize("mode", ("RGB", "L"))
 def test_matrix_xyz(mode: str) -> None:
     # Arrange
     im = hopper("RGB")
     im.info["transparency"] = (255, 0, 0)
-    assert im.mode == "RGB"
 
     # Act
     # Convert an RGB image to the CIE XYZ colour space
@@ -350,7 +357,6 @@ def test_matrix_identity() -> None:
         0, 1, 0, 0,
         0, 0, 1, 0,
     )  # fmt: skip
-    assert im.mode == "RGB"
 
     # Act
     # Convert with an identity matrix
diff --git a/src/_imaging.c b/src/_imaging.c
index e946bd37bb3..ee19f7f5dfc 100644
--- a/src/_imaging.c
+++ b/src/_imaging.c
@@ -1068,6 +1068,7 @@ _convert_matrix(ImagingObject *self, PyObject *args) {
                 m + 10,
                 m + 11
             )) {
+            PyErr_SetString(PyExc_TypeError, "matrix must be tuple of length 4 or 12");
             return NULL;
         }
     }