# What's wrong with my masking in this ImageMeasurements?

I want to find the mean color of an image's region defined by a polygon, and I tried this:

lena = Import["lena.tif"]
poly = Polygon[{{10, 10}, {120, 10}, {100, 100}}];

imgDim = ImageDimensions[lena];
imgBounds = Transpose[{{0, 0}, imgDim}];
mask = Image[Graphics[{White, poly}, Background -> Black,
ImageSize -> imgDim, PlotRange -> imgBounds]]



But when I run this I get an error

ImageMeasurements: Masking -> [the mask image] is not a known image measurements property.


ImageMeasurements takes the Masking option, so I expected there was something wrong with my masking image. But changing to Masking -> All still gives the same error. Is the documentation wrong?
(I'm on version 11.2)

poly = Polygon[{{10, 10}, {120, 10}, {100, 100}}];

Masking requires the graphical primitive instead your image. The sequence of arguments in ImageMeasurements have to be [image, measured value, options]. It does not work otherwise.