fix potential panic on compiling :-/1.

[ichiban]

No description provided.

[codecov]

Codecov Report

Merging #234 (cf64d11) into main (75737ad) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #234   +/-   ##
=======================================
  Coverage   97.42%   97.42%           
=======================================
  Files          21       21           
  Lines        6669     6669           
=======================================
  Hits         6497     6497           
  Misses        140      140           
  Partials       32       32           

Impacted Files	Coverage Δ
engine/clause.go	97.82% <100.00%> (ø)

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