Maxima weird unit problem

gistfile1.txt

(%i8) 1`(m/s) * 0`(1/s);
                                         1
(%o8)                                0 ` -
                                         s
(%i9) 0`(1/s) * 1`(m/s);
                                        m
(%o9)                               0 ` --
                                         2
                                        s
(%i10) (1`(m/s)) * (0`(1/s));
                                        m
(%o10)                              0 ` --
                                         2
                                        s
(%i11) (0`(1/s)) * (1`(m/s));
                                        m
(%o11)                              0 ` --
                                         2
                                        s