Cannot update identity column 'Index'. EF Core

Like the title says, I'm getting a error message while I try to update a entity in EF Core.

Here's the entity that I'm trying to update. Pay attention to the Index property tha's decorated with the Databasegenerated attribute.

I tried to decorate wthe property so a value would only be generated when a new entity is added, but without success.

Regarding to this link: http://ef.readthedocs.io/en/latest/modeling/generated-properties.html#value-generated-on-add

And finally the update method looks like this:

What am I doing wrong here?