help - r.mapcalc

Hi GRASS users,

I can't make r.mapcalc run properly. I always get the error message

WARNING: opencell: - illegal file name
OOPS can't create cell file <>

(to the comand : r.mapcalc result = soils + streams, for example)

It seams that G_open_cell_new is getting a Null file name. I tried
without =, spaces, with quotes, etc, but it refuses to work.

Any help would be much appreciated. Thanks in advance.

Pedro