Uncaught exception: TypeError: Cannot convert 'document.forms['artform']' to object

Error thrown at line 795, column 3 in <anonymous function>(http, data) in http://engine.loc/s2/_admin/js/admin.js:
    $(document.forms['artform'].elements['page[tags]'])
called from line 296, column 3 in <anonymous function>(data, textStatus, jqXHR) in http://engine.loc/s2/_admin/js/ajax.js:
    fCallback(jqXHR, data);
called via Function.prototype.apply() from line 2, column 29466 in <anonymous function: f.Callbacks>(b, f) in http://engine.loc/s2/_admin/js/jquery.js:
    if(c[l].apply(b,f)===!1&&a.stopOnFalse)
called from line 2, column 31004 in <anonymous function: fireWith>(b, c) in http://engine.loc/s2/_admin/js/jquery.js:
    d&&(i?a.once||d.push([b,c]):(!a.once||!e)&&n(b,c));
called from line 4, column 22304 in w(a, c, l, m) in http://engine.loc/s2/_admin/js/jquery.js:
    v.status=a,v.statusText=""+(c||w),o?h.resolveWith(e,[r,w,v]):h.rejectWith(e,[v,w,u]),v.statusCode(j),j=b,t&&g.trigger("ajax"+(o?"Success":"Error"),[v,d,o?r:u]),i.fireWith(e,[v,w]),t&&(g.trigger("ajaxComplete",[v,d]),--f.active||f.event.trigger("ajaxStop"))
called from line 4, column 34041 in <anonymous function: d>(a, e) in http://engine.loc/s2/_admin/js/jquery.js:
    m&&g(j,k,m,l)