coordinate system - How to reproject a raster to Winkel Tripel projection?

I'm trying to project a raster map (in ASCII grid format) from longlat to the Winkel Tripel projection. But I'm running into errors, for instance gdalwarp won't accept the proj-string.

$ gdalwarp -t_srs '+proj=wintri +ellps=WGS84 +datum=WGS84 +units=m +no_defs +towgs84=0,0,0' popd.asc popd_wintri.tif
ERROR 1: Translating source or target SRS failed:
+proj=wintri +ellps=WGS84 +datum=WGS84 +units=m +no_defs +towgs84=0,0,0

And if I try it in GRASS I get the following output

GRASS 6.5.svn (Wintri):~ > r.proj input=popd_2000AD location=Latlong output=test22 --o
Input Projection Parameters:  +proj=longlat +no_defs +a=6378137 +rf=298.257223563 +towgs84=0,0,0,0,0,0,0

Input Unit Factor: 1
Output Projection Parameters:  +proj=wintri +lat_0=0.0000000000 +lat_1=0.0000000000 +lon_0=20.0000000000 +a=6378137 +rf=298.257223563 +no_defs
Output Unit Factor: 1
WARNING: pj_transform() failed: non-convergent inverse meridional dist
(...)
ERROR: Input raster map is outside current region

I got the same error when I tried projectRaster() in R:

Error in projectRaster(test, wintri) : 
  error in pj_transform: non-convergent inverse meridional dist

On the GRASS website there's a screenshot of a raster map in Winkel Tripel-projection, so I'm guess it's possible to accomplish this in GRASS. Any help would be appreciated!