[igraph] Subgraph with members of communities with size > 20

[capitano . nemo]

I am trying to subgraph a graph (g) including only members of 
communities of size 20 or bigger.

This is my code but I am getting something wrong

     id_communities <- which(sizes(gc.fastgreedy)>20)
     subg <- induced.subgraph(g, which(membership(communities) == 
id_communities))

I get as warning

    Warning message:
    In membership(gc.fastgreedy) == id_communities :
      longer object length is not a multiple of shorter object length