Error: replacement has length zero

[ajileshparolla]

Hi, while running the RFmerge function I am getting this error quoted below.
"Error in obs.values[i] <- train.ts[day, grep(metadata[, id], names(train.ts))] : replacement has length zero".
Any help would be much appreciated. Also, I would like to have the output as NetCDF, so bring an option to specify output file format. Thanks in advance.

[fmanquehual]

Late, but it may be useful for someone else. In the metadata (ID,lon,lat) I had stations that were not present in the observed data file (x).