Keeps asking am I sure I throw an exception, what's wrong with this code?

public class GoKart {
  public static final int MAX_BARS = 8;
  private String mColor;
  private int mBarsCount;

  public GoKart(String color) {
    mColor = color;
    mBarsCount = 0;
  }

  public String getColor() {
    return mColor;
  }



  public void drive(int laps) {
    // Other driving code omitted for clarity purposes
   int lapAfter=MAX_BARS - laps;
   if (lapAfter < 0){
     throw new IllegalArgumentException("Not enough battery remains");
   }
    mBarsCount -= laps;
  }

  public void charge() {
    while (!isFullyCharged()) {
      mBarsCount++;
    }
  }

  public boolean isBatteryEmpty() {
    return mBarsCount == 0;
  }

  public boolean isFullyCharged() {
    return mBarsCount == MAX_BARS;
  }

}