api: Fix Derivative substitution

[mloubout]

Fixes @EdCaunt issue with substitutions.

[codecov]

Codecov Report

Merging #1942 (e0f8204) into master (e0f8204) will not change coverage.
The diff coverage is n/a.

❗ Current head e0f8204 differs from pull request most recent head c0c3352. Consider uploading reports for the commit c0c3352 to get more accurate results

@@           Coverage Diff           @@
##           master    #1942   +/-   ##
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  Coverage   87.82%   87.82%           
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  Files         213      213           
  Lines       35943    35943           
  Branches     5408     5408           
=======================================
  Hits        31568    31568           
  Misses       3868     3868           
  Partials      507      507           

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