Should be quite obvious.
Fixed in revisions #1498 (0.5-main) and #1515 (0.6-main).
The development of igraph has moved to github, so please do not comment on this bug here. You are of course welcome to comment on github, here: https://github.com/igraph/igraph/issues/162
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Fixed in revisions #1498 (0.5-main) and #1515 (0.6-main).