rolling total - Datesinperiod function help

[MX14]

Hi guys,

I was hoping to get some help with my measure. My goal is to create a measure that calculates the rolling average headcount for the past 12 months.I have created a headcount measure which is working fine, however when I try to use it to create the rolling total for past 12 months the rolling total measure returns error result.

Headcount = 

var headcount =
CALCULATE(
DISTINCTCOUNT(Data[ID Number]),
Data[Hire Month]<=SELECTEDVALUE('Calendar'[YearMonthNo]),
OR(
Data[Termination Month]=BLANK(),
Data[Termination Month]>SELECTEDVALUE('Calendar'[YearMonthNo])
),
ALL('Calendar')
)

return

SWITCH(TRUE(),
SELECTEDVALUE('Calendar'[YearMonthNo])<TODAY()-1,headcount,
BLANK())

Headcount last 12Month =

CALCULATE(

  AVERAGEX(Data,[Headcount]),

  DATESINPERIOD(

    'Calendar'[Date],SELECTEDVALUE('Calendar'[YearMonthShort]),-12,MONTH)

)

Thank you for your help!