The vote of a deceased Sinn Fein Assembly member may continue to be exercised even though the party has not filled his post.
Sinn Fein West Belfast MLA Michael Ferguson died in September
Michael Ferguson, 53, died in September after battling testicular cancer.
Sinn Fein has not been able to fill his vacancy, as none of the substitutes he nominated when elected in 2003 has been able to take up the job.
The vote can count towards the party's strength prior to the fresh election in March, the new St Andrews law says.
The clause will also apply if any other MLA's position became vacant.
Mr Ferguson was an assembly member for West Belfast. The father-of-four had been a councillor for the last 18 years.