Using geoR krige.conv

Hi,

I'm using the function krige.conv from package geoR but I'm getting a
warning that I do not understand:

Warning messages:
1: NaNs produced in: sqrt(variance)
2: NaNs produced in: sqrt(variance)

I'm performimg an ordinary kriging with a log transform (lambda=0).

Does anyone knows where this comes from ?

Thanks and regards

EJ