Ambiguous coordinates on X axis

[Hiroyuki Tomita <tomita@xxxxxxxxxxxxxxxxxxxxxxxx>]

Ferreters,

I have a question about a let command.
I am getting an NOTE as follows.
Why does the NOTE occur?

yes? let diff=slp[d=1] - slp2[d=2]
yes? fill diff[l=1]
 *** NOTE: Ambiguous coordinates on X axis: SLP[D=1] - SLP2[D=2]
yes?

But both my variables are identical in terms of grid and axes as 
follows.

yes? sho grid slp[d=1] slp2[d=2]
    GRID GOV1
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT       LATITUDE            73 r   90S                  90N
 normal    Z
 TIME      TIME               540 i   01-SEP-1957 00:00    01-AUG-2002 
00:00
    GRID GFM1
 name       axis              # pts   start                end
 LONGITUDE LONGITUDE          144mr   0E                   2.5W
 LATITUDE  LATITUDE            73 r   90S                  90N
 normal    Z
 TIME      TIME               540 i   01-SEP-1957 00:00    01-AUG-2002 
00:00
yes?


Thank you.

Hiroyuki Tomita