I'm trying to add a field in my attribute table to calculate square meters of some polygons using Area/1000000 but when I use the field calculator it says error in processing, check Geoprocessing Results window. So when I check this is the message: 
What can I do to fix this? The fields are compatible, they're both type "Double" an decimal places the same. I checked the source of the dataset and it's the same as the rest. What could be the issue here?