Importing Sales Invoices and updating dimensions

For a Customer we import Sales Invoices using a template based on the SIH import template.

Importing new, multiline invoices works fine. But when trying to update an invoice but adding an extra line, the error below occurs. A work around, is to first import the update to add the line without the line 'C' line (for the dimensions) and then import the same update again, this time with the 'C' line.

Does anyone have any idea why importing these dimensions gives an error?

-------------------- Record No. 3 (Line Number 9)
@X3.TRT/TRTX3CPT$adx (2058) Error 8 : Index Incorrect CCE1(-1)
Error 0 Line Number 0
Sage X3 errors

Field ITMREF(2) : Mandatory field <I-CCC09170>

Many thanks,

Gianni