operator ++ throwing: Error: replacement values for syntax template must not be null or undefined

Created by: NSilv

the following code:

operator ++ left 1 = (left, right) => #`${left}.concat(${right})`

let a = [1,2,3] 
let b = [4,5,6]
let c = a ++ b;

throws the following error:

Error: replacement values for syntax template must not be null or undefined
    at sanitizeReplacementValues (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\sweet.js\dist\load-syntax.js:64:11)
    at C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:3018:46
    at List.__iterate (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:2208:13)
    at IndexedIterable.mappedSequence.__iterateUncached (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:3017:23)
    at seqIterate (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:606:16)
    at IndexedIterable.IndexedSeq.__iterate (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:322:14)
    at IndexedIterable.toArray (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:4260:23)
    at new List (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:2067:62)
    at reify (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:3572:37)
    at List.map (C:\Users\User\Desktop\Nodejs\sweet_stuff\node_modules\immutable\dist\immutable.js:4403:14)