Whenever you are using the Function Module "REUSE_ALV_FIELDCATALOG_MERGE", you need to take care of the internal table you pass to the FM.
1) If you are passing a structure which is in SE11, then there would be no issue.
2) If you are passing an internal table, then make sure you have declared the components using "LIKE".
For instance,
Data : BEGIN of gt_tab occurs 0,
vbeln like vbeln,
posnr like posnr,
end of gt_tab.
Dont use "TYPE" addition.
Hope it helps.
Regards,
Sumit