Have you run the code to see ZERO for the error? Not sure, how it will help. Of course, the matrix is full of ZERO.
You got also this post related to similar stuff as always. Apparently, it is a common issue for everbody.
None of these posts, as far as I noticed, are a solution to get integral error for the background when the parameters has to be fixed. Thus. another way to approach the problem is needed as I came to conclusion.
I got another idea of creating a histogram for the background to represent, then pull the error from the background histogram with IntegralAndError() method at the moment. The picture below is the correct representation of the background. if you give any flexiblity to the parameters, we make a distortion from this base line which makes it pol2 or something else indeed.
I saw this post from @moneta. I also try to understand that.