content aware fill generates black fill

Hi! I am attempting to remove an object from approximately 1 second of video.. I tried with a piece of test footage and it worked flawlessly. I was super impressed with how painless it was and how well it worked. I am now trying to recreate the effect with a second clip and it is no longer working.. what i'm doing:

i have a clip with an object that i would like to remove.

i have a solid layer (gray)

i have masked around the object using the pen tool on the solid layer and am using track matte to mask the object with the solid layer

this all worked with my test footage, but now the CAF preview / fill target window only shows a black frame (the mask path is visible in the preview window), and when i hit "generate fill layer" the fill is also just black. 

i have tried inverting everything, generating a fill layer with the mask inverted, restarting AE, using an adjustment layer instead of a solid, copying the mask to the source clip... all with the same results.. no error messages, just a black fill.

any ideas?