In my experience, the text environment uses the first item, whether its a feature or pixel. I'm guessing the first "pixel" is null but all the others will return a valid response. I would also get the execution error when running it there. However, as shown in the image, I do see what the keys are in the $pixel dictionary.
The reason I was using "Raster.ItemPixelValue" was that it's shaded relief image. The "Raster.ServicePixelValue" key returned an array like ["69","176","45"], representing the red, green, and blue values.