Markers can be a plain string too

[alexandernst]

Check if markers is an object or a simple string before calling Object.keys(marker).map(function (key)

[alexandernst]

This could be a possible patch

      if(_.isString(marker)){
        return marker;
      }else{
        var str = Object.keys(marker).map(function (key) {
          if (STYLE_ATTRIBUTES.indexOf(key) > -1) {
            return key + ':' + marker[key] + '|';
          }
        }).join('');

        return str + marker.coords.join(',');
      }