SUM only accepts column ref

Hi there

I have DAX code for a one-column ("a") calculated table. See code and screenshot below.

SELECTCOLUMNS(
	ADDCOLUMNS(
		SUMMARIZE(
			FILTER(
				'Market data - Fx rates TTM'
				;'Market data - Fx rates TTM'[Fx ccy]="CNH"
				&& 'Market data - Fx rates TTM'[Spot/outright]="Spot"
				&& 'Market data - Fx rates TTM'[Quote type]="Mid"	
			)
			;'Market data - Fx rates TTM'[Fx date]
		)
		;"x=CNH";[EUR/CNH]
	)
	;"a"
	;[x=CNH]
)

What I want to do is SUM the values of this column, i.e. Measure = sum(calculated_table[a]).

BUT, I want to do this all _in memory_, without the need of an intermediate calculated table... therefore I went ahead and put this code into a SUM function directly 

sum a:= SUM(
	SELECTCOLUMNS(
		ADDCOLUMNS(
			SUMMARIZE(
				FILTER(
					'Market data - Fx rates TTM'
					;'Market data - Fx rates TTM'[Fx ccy]="CNH"
					&& 'Market data - Fx rates TTM'[Spot/outright]="Spot"
					&& 'Market data - Fx rates TTM'[Quote type]="Mid"	
				)
				;'Market data - Fx rates TTM'[Fx date]
			)
			;"x=CNH";[EUR/CNH]
		)
		;"a"
		;[x=CNH]
	)
)

However, this throws an error, stating that sum only takes a column reference...

How would I go about this?

Kind regards