grass - Issue with Sextante r.map calculator and modeler

I am trying to build a model with Sextante modeler and the r.mapcalculator algorithm but when I run the model I get the error message:

‘Could not load layer: ‘….’Check sextant log to look for errors in algorithm execution.’

The Sextant Log reads:

sextante.runalg("modeler:ndsi.","H:/NDSI/MODIS/20120607_SurfaceReflectance
   /0607_Processed Images/0607_b4_warp","H:/NDSI/MODIS/20120607_SurfaceReflectance
      /0607_Processed Images/0607_band6_warp","H:/NDSI/MODIS/test/NDSI")

The algorithm I have put into r.mapcalculator is a basic algorithm to calculate NDSI for snowcover analysis:

e.g. (amap-bmap)/(amap+bmap)

amap = band 4

bmap= band 6

I have tried on different computers as well as only using the r.mapcalculator through the Sextante Toolbox. Is there a different calculator I can use in my model? Is there a different way I should write out my algorithm?