The most direct route to identifying where the "error" arises would be to run your test program in the debugger, stepping through the machine code and checking the internal representations of the values at relevant points. It should be straight forward to identify which operation is in error and what the nature of the error is.

Having determined where the error is, you can write a correct function to replace the faulty one. Of course, adding explicit function calls to replace every typecast and implicit conversion will be non-trivial.