ofTrueTypeFont letterSpacing issue

#include "ofApp.h"

void ofApp::setup(){
    ofBackground(0, 255);
    ofTrueTypeFont::setGlobalDpi(72);
    font.load("Gotham Bold.otf", 22, true, true);
    font.setLetterSpacing(3);
}

void ofApp::draw(){
    
    ofPushMatrix();
    ofTranslate(20, ofGetHeight()/2);
    ofSetColor(255);
    font.drawString("OMNIJEWEL SATELLITE SPEAKERS", 0, 0);
    ofPopMatrix();
    
    ofDrawBitmapStringHighlight("Letter Spacing : "+ ofToString(lp,2), 20, 20);
}

void ofApp::keyPressed(int key){
    
    switch(key){
        case OF_KEY_UP:
            lp += 0.1;
            font.setLetterSpacing(lp);
            break;
            
        case OF_KEY_DOWN:
            lp -= 0.1;
            font.setLetterSpacing(lp);
            break;
    }
}