According to the SQL standard, floating point values that aren't regular values should be considered errors. The engine shouldn't produce NaN or Infinity:

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

                   ASIN COT
======================= =======================
                    NaN Infinity

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

               SUBTRACT
=======================
                    NaN

We have two options: detect the problem once it has occurred (and give a generic message) or check the parameters. Since we know the functions we offer, the latter seems feasible.