diff --git a/python_multipart/decoders.py b/python_multipart/decoders.py
index c4cdfb6..9aaabd9 100644
--- a/python_multipart/decoders.py
+++ b/python_multipart/decoders.py
@@ -60,6 +60,12 @@ def write(self, data: bytes) -> int:
         :param data: base64 data to decode
         """
 
+        # Line breaks and spaces may appear anywhere in Base64 (MIME wraps at
+        # 76 characters).  Drop them before counting, otherwise a chunk
+        # boundary falling inside a quartet is miscounted and the decode fails.
+        written = len(data)
+        data = data.translate(None, b"\r\n \t")
+
         # Prepend any cache info to our data.
         if len(self.cache) > 0:
             data = bytes(self.cache) + data
@@ -85,7 +91,7 @@ def write(self, data: bytes) -> int:
             self.cache[:] = b""
 
         # Return the length of the data to indicate no error.
-        return len(data)
+        return written
 
     def close(self) -> None:
         """Close this decoder.  If the underlying object has a `close()`
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index de220eb..dd762dc 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import base64
 import os
 import random
 import sys
@@ -692,6 +693,17 @@ def test_long_bad_split(self) -> None:
             self.d.write(second)
             self.assert_data(b"foobar")
 
+    def test_line_wrapped_input_any_split(self) -> None:
+        # MIME-style Base64 is wrapped with line breaks; the result must not
+        # depend on where the chunk boundaries fall.
+        raw = bytes(range(100))
+        wrapped = base64.encodebytes(raw)
+        for i in range(len(wrapped) + 1):
+            self.setUp()
+            self.assertEqual(self.d.write(wrapped[:i]), i)
+            self.d.write(wrapped[i:])
+            self.assert_data(raw)
+
     def test_close_and_finalize(self) -> None:
         parser = Mock()
         f = Base64Decoder(parser)