diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 76a27910da4d5f..0be61ff8a959f8 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -10,6 +10,7 @@
 import sys
 # Import _thread instead of threading to reduce startup cost
 from _thread import allocate_lock as Lock
+from itertools import repeat
 if sys.platform in {'win32', 'cygwin'}:
     from msvcrt import setmode as _setmode
 else:
@@ -1686,7 +1687,8 @@ def readall(self):
                 if addend < DEFAULT_BUFFER_SIZE:
                     addend = DEFAULT_BUFFER_SIZE
                 bufsize += addend
-                result[bytes_read:bufsize] = b'\0'
+                result.extend(repeat(0, addend))
+                assert len(result) == bufsize, "Should have expanded in size"
             assert bufsize - bytes_read > 0, "Should always try and read at least one byte"
             try:
                 n = os.readinto(self._fd, memoryview(result)[bytes_read:])
diff --git a/Misc/NEWS.d/next/Library/2025-01-31-19-07-31.gh-issue-129005.II6go0.rst b/Misc/NEWS.d/next/Library/2025-01-31-19-07-31.gh-issue-129005.II6go0.rst
new file mode 100644
index 00000000000000..260a8ba63a45d0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-01-31-19-07-31.gh-issue-129005.II6go0.rst
@@ -0,0 +1 @@
+:mod:`!pyio`: Fix expansion of buffer in _pyio.FileIO.readall