fix for [BUG] #411

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (25279a1) 81.79% compared to head (24a47ca) 81.80%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #433 +/- ## ======================================= Coverage 81.79% 81.80% ======================================= Files 80 80 Lines 4718 4720 +2 Branches 804 805 +1 ======================================= + Hits 3859 3861 +2 Misses 657 657 Partials 202 202 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `81.80% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/casact/chainladder-python/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [chainladder/methods/base.py](https://app.codecov.io/gh/casact/chainladder-python/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Y2hhaW5sYWRkZXIvbWV0aG9kcy9iYXNlLnB5) | `93.75% <100.00%> (+0.13%)` | :arrow_up: |

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