fg_batter_leaders splits using 'month' argument

[letta-9]

I am trying to filter batting leaders by splits (vs LHP or vs RHP). On the Fangraphs link, this is done by changing the 'month' to 13 (for vs LHP) and 14 (for vs RHP). However, I am getting an error when changing this argument in the fg_batter_leaders function. Other 'month' values are working properly. Is there a different way to pull the leaderboards by spilt?