lib/PublicInbox/LeiInput.pm | 2 ++

diff --git a/lib/PublicInbox/LeiInput.pm b/lib/PublicInbox/LeiInput.pm
index c258f824e8325101c80e55362ea6368b27434358..b6c2b6bbe8b77b30af2e273be04a548049a15937 100644
--- a/lib/PublicInbox/LeiInput.pm
+++ b/lib/PublicInbox/LeiInput.pm
@@ -30,6 +30,8 @@ 	L => sub {
 		my ($label) = @_;
 		length($label) >= $L_MAX and
 			return "`$label' too long (must be <= $L_MAX)";
+		$label =~ /[A-Z]/ and
+			return "`$label' must be lowercase";
 		$label =~ m{\A[a-z0-9_](?:[a-z0-9_\-\./\@,]*[a-z0-9])?\z} ?
 			undef : "`$label' is invalid";
 	},