An error creating a Function module. Can u help.

I have created a function group called zvendorgroup.

In that group i have created a simple function module called zvendfun.

The ABAP editor shows that the function module is syntatically correct but when i activate it an error pops up which is "The main program of the function ZVENDFUN does not begin with "FUNCTION-POOL".

How do i activate this function module that i have created. do i have to activate the function group that i have created. If yes then How?

Regards,

Vinod.