diff --git a/multipart/multipart.py b/multipart/multipart.py
index 137d6e7..158b7e6 100644
--- a/multipart/multipart.py
+++ b/multipart/multipart.py
@@ -1150,7 +1150,7 @@ def data_callback(name: CallbackName, end_i: int, remaining: bool = False) -> No
                 else:
                     # Check to ensure our boundary matches
                     if c != boundary[index + 2]:
-                        msg = "Did not find boundary character %r at index " "%d" % (c, index + 2)
+                        msg = "Expected boundary character %r, got %r at index %d" % (boundary[index + 2], c, index + 2)
                         self.logger.warning(msg)
                         e = MultipartParseError(msg)
                         e.offset = i
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index f55e228..b824f19 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -1057,7 +1057,12 @@ def test_bad_start_boundary(self) -> None:
         self.make("boundary")
         data = b"--boundaryfoobar"
         with self.assertRaises(MultipartParseError):
-            i = self.f.write(data)
+            self.f.write(data)
+
+        self.make("boundary")
+        data = b"--Boundary\r\nfoobar"
+        with self.assertRaisesRegex(MultipartParseError, "Expected boundary character %r, got %r" % (b"b"[0], b"B"[0])):
+            self.f.write(data)
 
     def test_octet_stream(self) -> None:
         files: list[File] = []