TypeError constructing page id: cannot concatenate 'str' and 'list' objects

OOPS-2018A54 TypeError: cannot concatenate 'str' and 'list' objects

 TypeError: cannot concatenate 'str' and 'list' objects

    Traceback (most recent call last):
  Module zope.publisher.publish, line 134, in publish
    result = publication.callObject(request, obj)
  Module lazr.restful.publisher, line 211, in callObject
    WebServicePublicationMixin, self).callObject(request, object)
  Module canonical.launchpad.webapp.publication, line 466, in callObject
    pageid = self.constructPageID(view, context)
  Module canonical.launchpad.webapp.servers, line 1171, in constructPageID
    pageid += ':' + op
TypeError: cannot concatenate 'str' and 'list' objects

HTTP method: OPTIONS
URL: https://api.launchpad.net/1.0/checkbox?ws.op=searchTasks