R igraph Cannot add edges, Invalid Vertex ID

Question

When trying to add edges to my graph using this line. I get "cannot add edges, Invalid vertex id"

graph <- add_edges(graph,c(as.character(Edgelist$A1[i]),as.character(Edgelist$A2[i])))

I have added my vertices to this graph with this code as follows

graph <- make_empty_graph(directed=FALSE)
for (i in 1:(length(unique(monthdata$AuthorID)))){
    graph <- add_vertices(graph,1,authorID=as.character(monthdata$AuthorID[i]))
}

I then attempt to add edges to this graph using the following nested loop

for(k in unique(monthdata$ThreadID)){
   temp = monthdata[(monthdata$ThreadID==k),]
   Edgelist = as.data.frame(t(combn(temp$AuthorID,2)))
   colnames(Edgelist) = c("A1","A2")
   for(i in 1:nrow(Edgelist)){
     graph <- add_edges(graph,c(as.character(Edgelist$A1[i]),as.character(Edgelist$A2[i])))
   }
}

Any help would be greatly appreciated I cannot seem to figure out what is giving me this "cannot add edges, invalid vertex id" error.

I would like to add this is NOT solvable by similarly titled posts. I have read and attempted their solutions but it doesn't seem to work.