[Ifeffit] why ss_2 is negative?

Dear all, I am doing model fitting using Artemis. My sample is uranium solution with an ionic liquid, 1-methoxyethyl-3-methyl imidazolium tetrafluoroborate (MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I don't know how can I fix this problem. Can you get a chance to take a look? My fitting is simple, only three single scattering pathes which were produced by Six Pack. The fitting file is attached. I really appreciate it if you could take a look and give some advice! Best regards! Hao