OpenStreetMap: two content types on map, marker-${type}.png -> marker-undefined.png when clustered

Hi OpenLayers-fans,

I'm having a problem with my OpenStreetMap in Drupal 6.
The markers for the content types are showing correctly via admin/build/openlayers/styles/ and marker-${type}.png. But when I activate "Cluster", all markers show as marker-undefined. I only want the clustered markers to show a different marker. Is there any way I can accomplish that?

Thanks!