[R-sig-Geo] gstat error

[Dave Depew]

Hi
I'm trying to run some universal kriging, and have not experienced this 
error before.
I've removed duplicate data locations using the remove.duplicates 
command. The data set runs fine if the formula is set as ordinary 
kriging, but adding in a predictor (which is already known at each grid 
location)
gives

"lufactor.c", line 207: singular matrix in function m_inverse()
Error in predict.gstat

Any ideas?