BUG: Trivial fix to #15447
 - [x] closes #15447
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

This is not really meant for merging because I don't understand that piece code and don't have time to understand it. But if you want to merge it anyway I can add a test and a whatsnew entry.
@toobaz you know that you can simply do this on your own fork. Please only do a pull request when you actually want to merge something (with tests and such). @jreback as I wrote, I have no problem in adding a test, that's not the point. The point is whether you want to apply the fix, given that I don't understand that code. well with a suite of tests sure

this type of indexing for sparse is barely tested

in fact what would be great would be a bunch of tests - see what works, and what doesn't u can xfail (and then when it's fixed it will be good)

we need systematic testing for this