Unlike ln(number), the log(base, number) function doesn't check its parameters. Examples of what happens with illegal params follow:

SQL> select log(2, 00) from rdb$database;

                    LOG
=======================
     -1.#INF00000000000

SQL> select log(2, -2) from rdb$database;

                    LOG
=======================
     -1.#IND00000000000

SQL> select log(0, 1) from rdb$database;

                    LOG
=======================
    -0.0000000000000000

SQL> select log(0, 2) from rdb$database;

                    LOG
=======================
    -0.0000000000000000

SQL> select log(-1, 2) from rdb$database;

                    LOG
=======================
     -1.#IND00000000000

SQL> select log(-1, 0) from rdb$database;

                    LOG
=======================
     -1.#IND00000000000

SQL> select log(-1, -1) from rdb$database;

                    LOG
=======================
     -1.#IND00000000000