Searching companyname in Partner

Apparently searching companyname of Partner is not possible.  Is there a way around this?

Here is my error:  "SSS_INVALID_SRCH_FILTER\nDetails: An nlobjSearchFilter contains invalid search criteria: companyname."

Here is my code: 

var filters = new Array();
filters[0] = new nlobjSearchFilter('companyname', null, 'contains', input.companyName);

var columns = [
    new nlobjSearchColumn('companyname')
];


nlapiLogExecution('DEBUG', 'readPartner()', 'searching for:  ' + input.searchfield + ' = ' + input.companyName + '; Filter  = ' + filters);
searchresults = nlapiSearchRecord('partner', null, filters, columns);