geoext legendpanel not displaying symbols

I am using geoext 1.1 with openlayers 2.11. I have a few point layers (WFS) with associated styles where I pass an image adress to 'externalGraphic' parameter like this 'http://ift.tt/1EXkAEV'. Works well. My problem is, when I make this parameter dynamic to use features attribute value like that "http://test.pt/images/" + '${cod}" + ".png" the layers symbols display the graphic cool but we cannot see them on legend. You can see an example on attached image. Somenone knows hot to fix that?

My legendPanel code:

var legendPanel = new GeoExt.LegendPanel({
    renderTo: "legenda",
    layerStore: mapPanel.layers,
    border: false,
    filter: function(record) {
        return !record.get("layer").isBaseLayer && record.get("layer").hideInLegend != true;                    
    },
    defaults: {
        style: 'padding:5px',                               
        baseParams: {
            FORMAT: 'image/png',
            LEGEND_OPTIONS: 'forceLabels:on;fontName=Verdana;fontSize:11'
        }
    }
});