fix(types): stricter types are now enforced

[bcoe]

Fixes #730

[codecov]

Codecov Report

Merging #733 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #733   +/-   ##
=======================================
  Coverage   99.17%   99.17%           
=======================================
  Files          15       15           
  Lines       15723    15723           
  Branches      955      958    +3     
=======================================
  Hits        15594    15594           
  Misses        126      126           
  Partials        3        3           

Impacted Files	Coverage Δ
src/instance.ts	99.82% <100.00%> (ø)

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