Workaround for undocumented breaking change in TS3.6

[aoberoi]

Summary

This PR adds a cast to workaround a breaking change that occurred in TypeScript v3.6. We may later want to change the implementation to one that doesn't use a cast. At this moment it is critical to unblock all builds.

Fixes #863

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #864 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #864   +/-   ##
=======================================
  Coverage   95.62%   95.62%           
=======================================
  Files          12       12           
  Lines         686      686           
  Branches      147      147           
=======================================
  Hits          656      656           
  Misses         11       11           
  Partials       19       19

Flag	Coverage Δ
#eventsapi	89.61% <ø> (ø)	⬆️
#interactivemessages	96.33% <ø> (ø)	⬆️
#webapi	98.5% <ø> (ø)	⬆️
#webhook	87.5% <ø> (ø)	⬆️

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