PLOT command ambiguous coordinates on T axis

[Steve Knox <stevek@xxxxxxxxxxxxxxxxxx>]

Why do I get the error on the first plot command but not when I simply 
repeat the command?
Any insights would be greatly appreciated.

ferret -gif
        NOAA/PMEL TMAP
        FERRET v5.50 (64bit CDF-No DODS)
        Solaris 5.8 - 01/23/03
        30-Jan-03 11:38

yes? use ncepM1979
yes? use ncepM1980
yes? plot/l=1:12 tmax[D=1,y=30:40@ave,x=-120:-60@ave]-tmax[d=2,y=30:40@ave,x=-120:-60@ave]
 *** NOTE: Ambiguous coordinates on T axis: TMAX[D=1,Y=30:40@AVE,X=-120:-60@AVE]-TMAX[D=2Y=30:40@AVE,X=-120:-60@AVE]
yes? plot/l=1:12 tmax[D=1,y=30:40@ave,x=-120:-60@ave]-tmax[d=2,y=30:40@ave,x=-120:-60@ave]
yes? frame/file=d.gif

The generated gif is ok.

Steve Knox
NREL
Colorado State University