Clip Disapearing when adding Alpha output -

Hi everyone,
I am new to Davinci (finally moving from Premiere) I am having trouble creating a masking transition. When I add the Alpha output in the colour tab and connect the blue line, the clip turns black and I can't see anything.
I suspect something needs to be inverted, just can't find the setting.
Appreciate any help!
I am new to Davinci (finally moving from Premiere) I am having trouble creating a masking transition. When I add the Alpha output in the colour tab and connect the blue line, the clip turns black and I can't see anything.
I suspect something needs to be inverted, just can't find the setting.
Appreciate any help!