My guess would be that the polymer has high far-UV absorbance.

In my opinion, there are two possible reasons for this: 1) the polymer itself has ellipticity in the UV region that you used to measure the CD for protein like Adam suggested, or 2) even though the polymer itself does not have CD signal, sometimes binding of the polymer to protein alters the chirality of the polymer, and this further changes the CD contribution from the polymer, aka induced CD~ Hope this helps~

Thanks Adams and Amy!