Brickwall filter : I still have a signal despite of setting bins to 0

[system64MC]

Hi! I wrote the following code :

import fftr, std/[math, sequtils]
import strutils

let
  # Square wave
  # signal = (0..1023).mapIt(complex64(if(it < 512): 1.0 else: -1.0))
  # Saw wave
  # signal = (0..1023).mapIt(complex64(2.0 * ((float(it * 12) / 1023.0) mod 1.0) - 1))
  # Sine wave
  signal = (0..1023).mapIt(complex64(sin(12 *  2 * PI * it.toFloat()/1024.0)))

  # getting freqs
var frequencies = fft(signal, false).mapIt(it / 1024.0)
echo frequencies.len

# We filter out every harmonic above 10...
const cutoff = 10
for i in countdown(1023 - cutoff, cutoff + 1):
  frequencies[i] = frequencies[i] * complex(0.0, 0.0)

var signalComplex = fft((0..1023).mapIt(frequencies[it]), true)
var signalAgain = signalComplex.mapIt(it.re)
assert(signalAgain.len <= 1024)
echo "Output signal : ", signalAgain

var outS16: array[1024, int16]
for i in 0..<1024:
  outS16[i] = (signalAgain[i] * 20000000000000000000.0).int16

import streams    
var f = newFileStream("outWave.bin", fmWrite)
if not f.isNil:
  f.write outS16
f.flush

I filter out every harmonic that is above the 10th. But when I filter a signal (here, a pure sine wave) with higher harmonics, I still have a signal, very silent, but I still get something instead of 0.

This is very problematic in case I want to normalize the signal.

Is that a bug or due to floating points doing suspicious things?

Thanks for your answer!