Problems with node names in Igraph with R - igraph

Question

I have a list of data.frames (dfList) from which I'd like to generate directed weighted networks in Igraph in R.
The edge-weight variable is "ValueUSD", while vertices are identified by numbers with the columns "Reporter" and "Partner".
Here it follows a function I prepared called "write.graphs" to be used with lapply on my "dfList".
write.graphs<-function(filename){
d<-graph.data.frame(filename[c("Reporter", "Partner")], directed=TRUE)
d <- set.edge.attribute(d, "weight", value=filename$ValueUSD)
}
graphs<-lapply(dfList, write.graphs)
Every thing work perfectly. If I check for vertex names I get:
graphs[1]$names
NULL
But troubles emerge when I want to use names in place of numbers to identify my vertices, using the corresponding columns "ReporterN" and "PartnerN" in each of my data.frames in dfLIst.
Here you can see how my data.frames look like:
dfList[1]
$`Aug 2014`
Reporter YearPeriod Year Period Commodity Partner NetWeightKg ValueUSD Price PartnerN ReporterN
1 76 201408 2014 8 150910 0 4472917 22028271 4.924811 World Brazil
2 76 201408 2014 8 150910 32 380891 1533948 4.027262 Argentina Brazil
3 76 201408 2014 8 150910 152 239776 1336057 5.572105 Chile Brazil
4 76 201408 2014 8 150910 251 289 2164 7.487889 France Brazil
5 76 201408 2014 8 150910 300 27592 170658 6.185054 Greece Brazil
6
This is the message I get:
> grafi<-lapply(dfList, scrivi.grafi)
There were 50 or more warnings (use warnings() to see the first 50)
> warnings()
Warning messages:
1: In graph.data.frame(filename[c("ReporterN", "PartnerN")], ... :
In `d' `NA' elements were replaced with string "NA"
2: In graph.data.frame(filename[c("ReporterN", "PartnerN")], ... :
In `d' `NA' elements were replaced with string "NA"
3: In graph.data.frame(filename[c("ReporterN", "PartnerN")], ... :
If it helps, I checked:
class(dfList[1]$PartnerN)
[1] "NULL"
Any suggestion? Can anyone explain me what happens?
Thanks a lot, Umberto.