CSS SLD generation for external graphics (url) fails

I am trying to use an external graphic to fill a polygon but I get the error

java.lang.Error: SOMEONE CODED THE X LINK NAMESPACE WRONG

Using the following style?

• { fill: url(grass_fill.png); fill-mime: 'image/png'; stroke: black; }

The image grass_fill.png is in the styles directory and works fine in the grass_poly.sld that ships with geoserver. I have tried quoting it etc.