Error when invoicing sales order AX 2012 FP -

We are getting error when we invoice a sales order in AX 2012 (FP). any idea what would cause this error?

4064.farnaz-error.png

When we try to invoice the Sales order in AX 2012 (fifo), AX is throwing the following error message

Ax.application.FormletterService.Run() in FormletterService.run.xpp:line 178

Appreciate any help with this.