--- a/mercurial/commands.py	Sun Oct 27 18:12:24 2019 +0100
+++ b/mercurial/commands.py	Sun Oct 27 12:30:59 2019 +0900
@@ -2267,7 +2267,9 @@
             fm.write(b'value', b'%s\n', value)
         else:
             fm.write(b'name value', b'%s=%s\n', entryname, value)
-        fm.data(defaultvalue=defaultvalue)
+        if formatter.isprintable(defaultvalue):
+            fm.data(defaultvalue=defaultvalue)
+        # TODO: no idea how to process unsupported defaultvalue types
         matched = True
     fm.end()
     if matched: