processing minim问题

import ddf.minim.*;
import ddf.minim.spi.*;
import ddf.minim.signals.*;
import ddf.minim.analysis.*;
import ddf.minim.ugens.*;
import ddf.minim.effects.*;
Minim minim;
AudioInput microphone;
void setup()
{
size(600,600);
minim=new Minim(this);
microphone=minim.getLineIn();
}
void draw()
{
float backgroundcol=microphone.mix.level()*255.0*10.0;
background(backgroundcol,backgroundcol,backgroundcol);
}
运行时显示nullpointerexception
有大神知道咋回事吗 processing版本为3.5.4