.at not setting value
#### Code Sample, a copy-pastable example if possible

```python
# ff= 20
# df['B'] = 0
# df.at[4, 'B'] = -ff/100
# df.at[4, 'B']
#0

```
#### Problem description

the function Dataframe.at is not setting the value if passed as variable, instead if it's passed as number, it works

#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
python           : 3.5.6.final.0
pandas           : 0.25.1

</details>
