syntax problem with r.mapcalculator

I created a raster map, "R20101001", using v.surf.rst.  The 
interpolation overshoots and dips below zero in a few areas.  I'd like 
to use r.mapcalculator, called from spgrass6, to replace all negative 
values with zeros.  But I can't seem to get the syntax right.  The 
following fails and produces the error message: "The system cannot find 
the file specified."

execGRASS("r.mapcalculator", flags = c("overwrite"),
    parameters=list(outfile="result", amap="R20100101", 
formula='if(A<0,0,A)'))

I would appreciate it if anyone can spot the mistake.