Newbie. Function module. default value not working.

Hi,

i have created a function module for a simple program with two numbers and operator as parameters and will give result based on the operators '+', '-', '*', '/'.

In the function module builder, i have created a field for the operator as fp_opr and given the default value as '+'. when i called this function from the program by giving operator , -,/ , it is working fine. but if i don't give any parameter, it is returning 0 value. since i have given the operator's default value in function module as '', i am expecting addition. but it is returning 0.

please help. What I am doing wrong?

thanks in advance.