I am attempting to update an existing merge request's labels by removing one and adding another. After connecting to the Gitlab server, I am trying the following:

forpingl.Project():if (p.id==json_project):formrinp.MergeRequest():if (mr.id==json_mr):final_labels= []forlabelinmr.labels:if (labelnotinattribs['remove']):final_labels.append(label)forlabelinattribs['add']:final_labels.append(u'%s'% (label))gl_mr.labels=final_labelsgl_mr.save()

I am getting the error:GitlabUpdateError: 400: Source branch cannot be changed

Is this the proper way to update an existing merge request or am I doing something wrong?