fix missing type for channel parameter

[merrx]

Fixes #2546

This PR adds the missing type statement.

[codecov]

Codecov Report

Base: 99.52% // Head: 99.52% // No change to project coverage 👍

Coverage data is based on head (def35d4) compared to base (e975ab9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2570   +/-   ##
=======================================
  Coverage   99.52%   99.52%           
=======================================
  Files          32       32           
  Lines        1697     1697           
=======================================
  Hits         1689     1689           
  Misses          8        8           

Impacted Files	Coverage Δ
src/program.js	98.57% <ø> (ø)

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