t.Errorf("test %d, unexpected WriteHeader error: %v", i, err)
		}
		if err := tw.Close(); err != nil {
			t.Errorf("test %d, unexpected Close error: %v", i, err)
		}

		// The prefix field should never appear in the GNU format.
		var blk block
		copy(blk[:], b.Bytes())
		prefix := string(blk.toUSTAR().prefix())
		prefix, _, _ = strings.Cut(prefix, "\x00") // Truncate at the NUL terminator