In IAR Embedded Workbench for AVR, after turning on fill (Project Options -> Linker -> Processing Tab) you get:
Fatal Error[e38]: Option -H must not be defined more than once: -H1895
Disable "Initialize unused interrupt vectors with RETI instructions." (Project Options -> General Options -> System).
Filling the interrupt vector table with RETI uses the same -H option, and only one is allowed.