postgis - ST_AsRaster for polygon

Having a hard time to get a raster from a polygon

ST_AsRaster(ST_Buffer(ST_GeomFromText('POLYGON((-30 40, -20 30, -25 20, -23 10, -30 40))', 4326), 50),ARRAY['8BUI'],ARRAY[118]);

This line is giving me an error : No function matches the given name and argument types. You might need to add explicit type casts.

Answer

You are missing pixel dimensions:

SELECT ST_AsRaster(ST_Buffer(ST_GeomFromText('POLYGON((-30 40, -20 30, -25 20, -23 10, -30 40))', 4326), 50),100,100,ARRAY['8BUI'],ARRAY[118]);