[igraph] How to identify negative vertices in graph

[capitano . nemo]

I have a problem with the function bipartite.projection() which returns

Error in bipartite.projection(igraph_bi) :
  At structure_generators.c:84 : Invalid (negative) vertex id, Invalid 
vertex id

when trying to project my bipartite graph.

Unfortunately my graph is very large (IGRAPH DN-B 154626 448387 -- ) so 
I can't easily reproduce the error in a minimum working example to 
share.

What I tried to do was to have a look at the dataframes that I used to 
create the graph object in the first place with

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

I checked the "id" column of my "nodes" df along with the "from" column 
of my "edges" df for negative values. (By the way, both "id" and "from" 
columns are character vectors, not numeric). Indeed I found 25 
"negative" strings in my "id" column of the "nodes" df--it depends on 
the first character, non-alphanumerics make for a negative value of the 
string. But still after removing all the negative strings, and after 
double-checking again both "id" and "from" columns, I keep getting the 
same error from bipartite.projection()

Error in bipartite.projection(igraph_bi) :
  At structure_generators.c:84 : Invalid (negative) vertex id, Invalid 
vertex id

Now my problem is to understand how to find where the negative values 
that trigger the error are hidden.

Is the value of the column "id" used as id of the node without any 
transformation (and same for the column "from" for edges) by the 
function graph.data.frame?

How can I get the id values out of the graph object to check for 
negative values?