Add Shipping Type in AR Invoice in Crystal Report

Dear Experts,

I'm designing the layout by Crystal Report. I want to add Shipping Type in AR Invoice. I used the ready format provided in version 8.8. However, there is no Shipping Type in the given format. So, I pulled in the OSHP table and link TrnspCode to OINV's TrnspCode.

This form works only if I select a shipping type from the drop-down menu in the AR Invoice. If I leave the shipping type empty, this form will generate a blank data.

Where did I go wrong?