Help with java treehouse challenge

I am being tasked this:

Okay, so let's throw an IllegalArgumentException from the drive method if the requested number of laps cannot be completed on the current battery level. Make the exception message "Not enough battery remains".

So i added this part to my 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() { drive(1); }

public void drive(int laps) { // Other driving code omitted for clarity purposes mBarsCount -= laps;

if (laps > mBarsCount) {

 throw new IllegalArgumentException("Not enough battery remains"); 
}

}

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

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

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

}

it does not give me a error it just says: Bummer! Make sure you throw the exception before change the mBarsCount field.

please help my understand why this does not work