PageRank: NoMethodError: undefined method `-' for nil:NilClass

[maia]

There's a problem with PageRank calculation: whenever you add a source that's not the destination of any other source, you end up with:

lib/graph-rank/page_rank.rb:67:in `block in convergence'
lib/graph-rank/page_rank.rb:66:in `each'
lib/graph-rank/page_rank.rb:66:in `convergence'
lib/graph-rank/page_rank.rb:40:in `calculate'

E.g. try:

pr = GraphRank::PageRank.new
pr.add 1,2
pr.add 3,1
pr.calculate

[dripisforever]

Hey @louismullie , what does may cause this issue?