R error "sum not meaningful for factors"

I have a file called rRna_RDP_taxonomy_phylum with the following data :

364  "Firmicutes"            39.31
244  "Proteobacteria"        26.35
218  "Actinobacteria"        23.54
65   "Bacteroidetes"         7.02
22   "Fusobacteria"          2.38
6    "Thermotogae"           0.65
3     unclassified_Bacteria  0.32
2    "Spirochaetes"          0.22
1    "Tenericutes"           0.11
1     Cyanobacteria          0.11

And I'm using this code for creating a pie chart in R:

if(file.exists("rRna_RDP_taxonomy_phylum")){
    family <- read.table ("rRna_RDP_taxonomy_phylum", sep="\t")
    piedat <- rbind(family[1:7, ],
                as.data.frame(t(c(sum(family[8:nrow(family),1]),
                                "Others",
                                sum(family[8:nrow(family),3])))))
    png(file="../graph/RDP_phylum_low.png", width=600, height=550, res=75)
    pie(as.numeric(piedat$V3), labels=piedat$V3, clockwise=TRUE, col=graph_col, main="More representative Phyliums")
    legend("topright", legend=piedat$V2, cex=0.8, fill=graph_col)
    dev.off()
    png(file="../graph/RDP_phylm_high.png", width=1300, height=850, res=75)
    pie(as.numeric(piedat$V3), labels=piedat$V3, clockwise=TRUE, col=graph_col, main="More representative Phyliums")
    legend("topright", legend=piedat$V2, cex=0.8, fill=graph_col)
    dev.off()
}

I've been using this code for different datafiles and it works fine, but with the file presented adobe it crash returning the following message:

Error in Summary.factor(c(6L, 2L, 1L), na.rm = FALSE) : 
  sum not meaningful for factors
Calls: rbind -> as.data.frame -> t -> Summary.factor
Execution halted

I need to understand why it crash with this file and if there's any way to prevent this kind of errors.

Thanks!