[igraph] How to use contract.vertices() on empty string

[capitano . nemo]

I have a network I would like to contract over the attribute 'category'. 
BUT some vertices take the value "" (empty string) for the attribute 
category. This makes contract.vertices() gives the error:

  Error in contract.vertices(pg$proj2, category_attr) :
    At structure_generators.c:84 : Invalid (negative) vertex id, Invalid 
vertex id

What's the best strategy to avoid this error? (Note that I can't replace 
'category' with 'NA', which for the vertices type=FALSE takes another 
meaning in my network.)

I was able to replicate my situation in a MWE

id <- c("www.user_a.com","www.user_b.com",
        "www.user_c.com","www.user_d.com",
        "www.user_e.com","www.thread_1.com",
        "www.thread_2.com","www.thread_3.com",
        "www.thread_4.com","www.thread_5.com")
type <- c(FALSE,FALSE,
          FALSE,FALSE,
          FALSE,TRUE,
          TRUE,TRUE,
          TRUE,TRUE)
name <- c("user_a","user_b",
          "user_c","user_d",
          "user_e","thread_1",
          "thread_2","thread_3",
          "thread_4","thread_5")
category <- c(NA,NA,
              NA,NA,
              NA,"Dev",
              "Traffic","Energy",
              "","Dev")

nodes <- data.frame(id,name,type,category)

from <- c("www.user_a.com","www.user_b.com",
          "www.user_c.com","www.user_d.com",
          "www.user_e.com","www.user_d.com",
          "www.user_c.com","www.user_b.com",
          "www.user_a.com","www.user_e.com",
          "www.user_c.com","www.user_e.com")
to <-c("www.thread_1.com","www.thread_2.com",
       "www.thread_3.com","www.thread_4.com",
       "www.thread_5.com","www.thread_1.com",
       "www.thread_2.com","www.thread_3.com",
       "www.thread_4.com","www.thread_5.com",
       "www.thread_5.com","www.thread_1.com")

edges <- data.frame(from,to)

g <- graph.data.frame(edges, directed=TRUE, vertices=nodes)

pg <- bipartite.projection(g)

category_attr <- get.vertex.attribute(pg$proj2, "category")

map = setNames(c(1:4), c("Dev","Traffic","Energy",""))

category_attr <- map[unlist(category_attr)]

igraph_category <- contract.vertices(pg$proj2, category_attr)

## Error in contract.vertices(pg$proj2, category_attr) :
## At structure_generators.c:84 : Invalid (negative) vertex id, Invalid 
vertex id