I just had the same message with the following code (in IcedCoffeeScript):
f = (err,cb) ->
cb null, true
await f defer err, res
console.log err if err
This seemed to me like regular ICS code. I unfolded the await-defer construct to regular CoffeeScript:
f (err,res) ->
console.log err if err
What really happend was that I tried to pass 1 callback function( with 2 parameters ) to function f expecting two parameters, effectively not setting cb inside f, which the compiler correctly reported as undefined is not a function.
The mistake happened because I blindly pasted callback-style boilerplate code. f doesn't need an err parameter passed into it, thus should simply be:
f = (cb) ->
cb null, true
f (err,res) ->
console.log err if err
In the general case, I'd recommend to double-check function signatures and invocations for matching arities. The call-stack in the error message should be able to provide helpful hints.
In your special case, I recommend looking for function definitions appearing twice in the merged file, with different signatures, or assignments to global variables holding functions.