TMS320F28035: About the call function _IQNexp() in the IQMath

Dear Sir.

As the figure is shown below, I call the _IQ18exp() function. The result of _IQ18exp(Is_Comm_Umin_t), Is_Comm_Umin_2, is no change whatever the Is_Comm_Umin_t is changed where Is_Comm_Umin_t and Is_Comm_Umin_2 are both declared as _IQ18 format. What's wrong?

Thanks

Lewis