Github user uce commented on the pull request:
https://github.com/apache/flink/pull/999#issuecomment-129357904
Thanks for the contribution. I think both Max and Fabian have a valid
point. This will lead to unexpected behaviour like `a.union(b)` working, but
then `b.union(a)` throwing a `NullPointerException`.
If you want, you can add an extra null check with a better error message in
case of a `null`. Otherwise, I think we can close this PR.
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