I am following the GAGE pathway analysis workflow from the 'gene_exp.diff' file produced by Cuffdiff v1.3.0 (part 7.5 here) and get the following error:

# Cuffdiff fork
cuff.res=read.delim(file="/home/user/gene_exp.diff", sep="\t")
#notice the column name special character changes. The column used to be
#cuff.res$log2.fold_change. for older versions of Cufflinks.
cuff.fc=cuff.res$log2.FPKMy.FPKMx.
gnames=cuff.res$gene
sel=gnames!="-"
gnames=as.character(gnames[sel])
cuff.fc=cuff.fc[sel]
names(cuff.fc)=gnames

Error in names(cuff.fc) = gnames : attempt to set an attribute on NULL

What does the error mean? I checked my 'gene' column on the 'gene_ex.diff' but I don't have any blank field (or the problem is not related to this column at all?)