WCS: server fails with a small coverage

[Jos é Vte Higón]

Hi list,
I'am using mapserver wcs 4.6.0, when I request a coverage with a small size
(an extent quite big),
the server returns this error:

msDrawMap(): Image handling error. Unable to initialize image.
msPrepareImage(): General error message. Image dimensions not specified.
msCalculateScale(): General error message. Invalid image extent,
minx=-1.000000, miny=-1.000000, maxx=-1.000000, maxy=-1.000000.
mapserv(): Web application error. No way to generate map extent.

This is the request:
http://gruposigp2/cgi-bin/wcs?COVERAGE=quickbird3&CRS=EPSG:32630
&BBOX=742806.4,4264938.3,753600.4,4268018.3
&bands=1,2,3&FORMAT=GEOTIFF_INT16&WIDTH=18&HEIGHT=5

In order to prevent this error, I have tried to filter this layer when the
scale is bigger than 1000000 meters:
...
UNITS METERS
MAXSCALE 1000000
...
but, the error is still happening!

What I am doing wrong?
Thanks in advance