In R: how to extract the first xmlValue from multiple xmlValues in the XML format file

I have the following document, which is a XMLNodeSet, I would like to extract the first xml value, if the leaf node has no values then it should give NA value. The result I want should be this: "11557040" "23667301" "NA"

but after using sapply(doc, xmlValue) it give the result like the follows:

[1] "115570409626101208908"  "2366730130010285360545" "\n\t"     

Any help is appreciated.

> doc
[[1]]
<CompoundIDList>
<int>11557040</int>
<int>962</int>
<int>6101</int>
<int>208908</int>

[[2]]
<CompoundIDList>
<int>23667301</int>
<int>3001028</int>
<int>5360545</int>
</CompoundIDList> 

[[3]]
<CompoundIDList>
</CompoundIDList> 

attr(,"class")
[1] "XMLNodeSet"`