Fix inference failure on julia 1.6

[timholy]

No description provided.

[codecov]

Codecov Report

Merging #12 (6d70ac4) into master (de77599) will increase coverage by 4.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   29.44%   33.48%   +4.04%     
==========================================
  Files           1        1              
  Lines         557      642      +85     
==========================================
+ Hits          164      215      +51     
- Misses        393      427      +34     

Impacted Files	Coverage Δ
src/RegisterOptimize.jl	33.48% <100.00%> (+4.04%)	⬆️

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