Some text-sizes of labels are not working in TileMill

Question

I am using TileMill to create some offline tiles. Currently I have problem that some of the text sizes are not working and they are displayed as weird signs. Do you have any idea what could be wrong?

  #waterways {
  line-smooth: 1;
  line-join: round;
  line-cap: round;

  //label styling and placement
  text-name: [name];
  text-face-name: 'Calibri Italic';
  text-fill: @water_labels;
  text-placement: line; 
  text-halo-fill: fadeout(white, 0%);
  text-halo-radius: 1.8;

  [type = 'river'] {line-color: @water; line-width:12; 
    [zoom=15] {text-size: 15;}
    [zoom=16] {text-size: 16;}
    [zoom=17] {text-size: 17;}
    [zoom=18] {text-size: 18;}
    [zoom=19] {text-size: 19;}
  }
}

Picture a) text-size: 18

Picture b) text-size: 17

Answer 1

Finally, I solved this problem by using my own fonts and this problem immediately stopped:

Map {
  background-color: @background;
  font-directory: url(./fonts); 
}