Fix FID computation for non equal size

[SkafteNicki]

What does this PR do?

Fixes #1027

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1028 (7e8ae8f) into master (6f5ac1e) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1028   +/-   ##
======================================
- Coverage      95%     95%   -0%     
======================================
  Files         180     180           
  Lines        7823    7824    +1     
======================================
- Hits         7430    7419   -11     
- Misses        393     405   +12