Using SUMMARIZE as a variable in NETWORKDAYS expression

Good afternoon,

Please could someone help me with an issue I am struggling with.  I have a Calendar dimension table that contains flags ("Y") for all UK Bank Holidays.  

I am trying to create an expression where:

1. I define a variable that returns the filtered column of dates that are public holidays (to achieve this I am using the SUMMARIZE function)
2. Use this variable in the NETWORKDAYS expression as the <holiday> parameter

The formula (which is not working) is:

NetWorkDays Summarize variable= 
VAR colHolidays = 
SUMMARIZE(FILTER(dimCalendar,dimCalendar[flgHoliday]="Y"),dimCalendar[Date])
RETURN
NETWORKDAYS(MAX(Projects[start_date]),MAX(Projects[end_date]),1,colHolidays)

The reason I thought this would work is I was able get the correct results by doing this in two stages:

First by creating a table using that SUMMARIZE formula

Table = 
SUMMARIZE(FILTER(dimCalendar,dimCalendar[flgHoliday]="Y"),
dimCalendar[Date])

And then I referenced that table in a NETWORKDAYS expression and it worked:

NetworkDays SUMAMRIZE Table = NETWORKDAYS(MAX(Projects[start_date]),MAX(Projects[end_date]),1,'Table')

Here is a screenshot of the table of results:

Please can you help me fix my variable definition?

Kind regards,

Paul