For correcting field errors in the Twente wiggler small metal shims were placed on the pole faces. When applied to correct errors in the on-axis field a large increase in the errors in the off-axis field was found. When instead errors in the off-axis field are corrected, the errors in the on-axis field are reduced as well. Only small corrections were locally required to make the errors in the on-axis field acceptable. The focussing strength of the wiggler has been measured in both transverse planes and was found to lead to a slightly elliptical shape for a matched electron beam. Longitudinal phase errors have been measured for the Twente wiggler and are negligible.