Function returning blank in DAX

I actually managed to solve it, its because when I filter something with my slicer it also filters the summarized table so I needed to add removefilters() within my DAX code like so:

testing = 
VAR extractCampaignYear = IF (ISFILTERED('Campaign Overview'[campaign]),MIN('Campaign Overview'[previousExtractedYear]),0)

RETURN
CALCULATE(
SUMX(
FILTER(
SUMMARIZE (
'Campaign Overview',
'Campaign Overview'[campaign],
"extractedCampaign",TRIM(MIN('Campaign Overview'[extractedCampaignName])),
"campaignYear", MIN('Campaign Overview'[extractedYearFromCampaign]),
"Sales", SUM ( 'Campaign Overview'[sales] )
),
[campaignYear] = extractCampaignYear), [Sales]),
REMOVEFILTERS()
)