lib/PublicInbox/IMAP.pm |  7 +++++--
 t/imapd.t               | 11 +++++++++++

diff --git a/lib/PublicInbox/IMAP.pm b/lib/PublicInbox/IMAP.pm
index e726307a6785ba98794b06aac66302f323ee4bbf..0452d6df9373bafbd749a8757ae4f655363d9193 100644
--- a/lib/PublicInbox/IMAP.pm
+++ b/lib/PublicInbox/IMAP.pm
@@ -862,14 +862,17 @@ 		} elsif (uc($tag // '') eq 'DONE' && !defined($req)) {
 			cmd_done($self, $tag);
 		} else { # this is weird
 			auth_challenge_ok($self) //
-				"$tag BAD Error in IMAP command $req: ".
-				"Unknown command\r\n";
+					($tag // '*') .
+					' BAD Error in IMAP command '.
+					($req // '(???)').
+					": Unknown command\r\n";
 		}
 	};
 	my $err = $@;
 	if ($err && $self->{sock}) {
 		$l =~ s/\r?\n//s;
 		err($self, 'error from: %s (%s)', $l, $err);
+		$tag //= '*';
 		$res = "$tag BAD program fault - command not performed\r\n";
 	}
 	return 0 unless defined $res;
diff --git a/t/imapd.t b/t/imapd.t
index 45ee401a1e0cfa598d10c07dde11da7a4b50c8f5..11b56b09dbeb0113d335e5e73e72387a5f8f1eb5 100644
--- a/t/imapd.t
+++ b/t/imapd.t
@@ -367,11 +367,22 @@ ok($mic->close, 'CLOSE works');
 ok(!$mic->close, 'CLOSE not idempotent');
 ok($mic->logout, 'logged out');
 
+{
+	my $c = tcp_connect($sock);
+	$c->autoflush(1);
+	like(<$c>, qr/\* OK/, 'got a greeting');
+	print $c "\r\n";
+	like(<$c>, qr/\A\* BAD Error in IMAP command/, 'empty line');
+	print $c "tagonly\r\n";
+	like(<$c>, qr/\Atagonly BAD Error in IMAP command/, 'tag-only line');
+}
+
 $td->kill;
 $td->join;
 is($?, 0, 'no error in exited process');
 open my $fh, '<', $err or BAIL_OUT("open $err failed: $!");
 my $eout = do { local $/; <$fh> };
 unlike($eout, qr/wide/i, 'no Wide character warnings');
+unlike($eout, qr/uninitialized/i, 'no uninitialized warnings');
 
 done_testing;