length(:productID) > length(':productID')

length(:productID) > max size of a variable used in the 'custom function'

SQL> CREATE OR REPLACE FUNCTION fun_test (par_1 IN CHAR)
  2    RETURN NUMBER
  3  IS
  4    l_length NUMBER(1);         --> note local variable's size!
  5  BEGIN
  6    l_length := LENGTH(par_1);
  7    RETURN (l_length);
  8  END;
  9  /

Function created.

SQL>
SQL> SELECT fun_test('len < 10') result FROM dual;

    RESULT
----------
         8

SQL>
SQL> SELECT fun_test('length is > 10') result FROM dual;
SELECT fun_test('length is > 10') result FROM dual
       *
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: number precision too large
ORA-06512: at "SCOTT.FUN_TEST", line 6


SQL>