diff --git a/backup.py b/backup.py
index 5a68bb5..a4118a4 100755
--- a/backup.py
+++ b/backup.py
@@ -257,10 +257,12 @@ class Backup(object):
                     os.makedirs(tmpdir, exist_ok = True)
                     os.chmod(tmpdir, mode = 0o0700)
                     for idx, prep in enumerate(repo['prep']):
-                        exec_tmp = tempfile.mkstemp(prefix = '_borgextend.',
-                                                    suffix = '._tmpexc',
-                                                    text = True,
-                                                    dir = tmpdir)[1]
+                        exec_fd, exec_tmp = tempfile.mkstemp(prefix = '_borgextend.',
+                                                             suffix = '._tmpexc',
+                                                             text = True,
+                                                             dir = tmpdir)[1]
+                        # D'oh. I need to close the FD before messing with it.
+                        os.close(exec_fd)
                         os.chmod(exec_tmp, mode = 0o0700)
                         with open(exec_tmp, 'w') as f:
                             f.write(prep)