[provide general introduction to the issue and why it is relevant to this repository]

## Context of the issue

[provide more detailed introduction to the issue itself and why it is relevant]

[the remaining entries are only necessary if you are reporting a bug]

## Process to reproduce the issue

[ordered list the process to finding and recreating the issue, example below]

1. User creates TPOT instance
2. User calls TPOT `fit()` function with training data
3. TPOT crashes with a `KeyError` after 5 generations

## Expected result

[describe what you would expect to have resulted from this process]

## Current result

[describe what you currently experience from this process, and thereby explain the bug]

## Possible fix

[not necessary, but suggest fixes or reasons for the bug]

## `name of issue` screenshot

[if relevant, include a screenshot]
