diff --git a/t/t0021/rot13-filter.pl b/t/t0021/rot13-filter.pl
index 37cecd8654..f31ff595fe 100644
--- a/t/t0021/rot13-filter.pl
+++ b/t/t0021/rot13-filter.pl
@@ -96,7 +96,8 @@ sub packet_bin_read {
 sub packet_txt_read {
 	my ( $res, $buf ) = packet_bin_read();
 	unless ( $res == -1 or $buf eq '' or $buf =~ s/\n$// ) {
-		die "A non-binary line MUST be terminated by an LF.";
+		die "A non-binary line MUST be terminated by an LF.\n"
+		    . "Received: '$buf'";
 	}
 	return ( $res, $buf );
 }