diff -r 40b6d25ed2d5 -r 0efdfb57b05c mercurial/config.py
--- a/mercurial/config.py	Tue Oct 03 16:59:17 2017 -0700
+++ b/mercurial/config.py	Tue Oct 03 12:00:07 2017 -0700
@@ -177,6 +177,7 @@
                 if sections and section not in sections:
                     continue
                 if self.get(section, name) is not None:
+                    self._data[section] = self._data[section].preparewrite()
                     del self._data[section][name]
                 self._unset.append((section, name))
                 continue