Explicitly mark nullable parameter

[cedric-anne]

An occurence was missed in #126.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (a6cd725) to head (244b34b).
Report is 1 commits behind head on 5.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.1     #128   +/-   ##
=========================================
  Coverage     99.72%   99.72%           
  Complexity      118      118           
=========================================
  Files             7        7           
  Lines           365      365           
=========================================
  Hits            364      364           
  Misses            1        1           

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[phil-davis]

Thanks, I wonder how I missed that.