diff --git a/git/objects/util.py b/git/objects/util.py
index 64cdf71fd..1fa3b9df0 100644
--- a/git/objects/util.py
+++ b/git/objects/util.py
@@ -100,9 +100,11 @@ def mode_str_to_int(modestr: Union[bytes, str]) -> int:
         module regarding the rwx permissions for user, group and other, special flags
         and file system flags, such as whether it is a symlink.
     """
+    if isinstance(modestr, bytes):
+        # A byte is an int. int(b"4"[0]) is 52, so b"100644" became 0o6767524.
+        modestr = modestr.decode("ascii")
     mode = 0
     for iteration, char in enumerate(reversed(modestr[-6:])):
-        char = cast(Union[str, int], char)
         mode += int(char) << iteration * 3
     # END for each char
     return mode
diff --git a/test/test_util.py b/test/test_util.py
index c6e68cf51..46417cc39 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -22,6 +22,7 @@
 from git.objects.util import (
     altz_to_utctz_str,
     from_timestamp,
+    mode_str_to_int,
     parse_actor_and_date,
     parse_date,
     tzoffset,
@@ -760,3 +761,10 @@ def test_remove_password_from_command_line(self):
         redacted_cmd_6 = remove_password_if_present(cmd_6)
         assert authorization not in " ".join(redacted_cmd_6)
         assert "http.extraHeader=Authorization: *****" in redacted_cmd_6
+
+
+def test_mode_str_to_int_accepts_bytes():
+    assert mode_str_to_int("100644") == 0o100644
+    assert mode_str_to_int(b"100644") == 0o100644
+    assert mode_str_to_int("644") == 0o644
+    assert mode_str_to_int(b"120000") == 0o120000