BUG: caught typeError in series.at (#25506)
- [x] closes #25506
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=h1) Report
> Merging [#25533](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd49d2f2af5c44d4f96031757743ba83e6b29408?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25533/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25533      +/-   ##
==========================================
- Coverage   91.75%   91.74%   -0.01%     
==========================================
  Files         173      173              
  Lines       52960    52960              
==========================================
- Hits        48595    48590       -5     
- Misses       4365     4370       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <100%> (-0.02%)` | :arrow_down: |
| #single | `41.73% <0%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.68% <100%> (ø)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `93.3% <0%> (-2.24%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.06%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=footer). Last update [bd49d2f...24ebb20](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=h1) Report
> Merging [#25533](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd49d2f2af5c44d4f96031757743ba83e6b29408?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25533/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25533      +/-   ##
==========================================
- Coverage   91.75%   91.74%   -0.02%     
==========================================
  Files         173      173              
  Lines       52960    52960              
==========================================
- Hits        48595    48589       -6     
- Misses       4365     4371       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <100%> (-0.02%)` | :arrow_down: |
| #single | `41.71% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.68% <100%> (ø)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `93.3% <0%> (-2.24%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25533/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.06%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=footer). Last update [bd49d2f...ceb13f8](https://codecov.io/gh/pandas-dev/pandas/pull/25533?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @jopenmolles! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-03-04 20:06:29 UTC @mroeschke @jreback I've added a test, although I was not sure if it should be placed in `test_timezones.py` (I placed it here now) or in one of the indexing tests (e.g. `test_indexing.py`). thanks @jopenmolles 