Java: Use EasyMock to mock String.getBytes("UTF-8")

Following is the code in actual Java class:

try {
   byte[] encodedBytes = Base64.encodeBase64(queryParams.getBytes("UTF-8"));
} catch (UnsupportedEncodingException e) {
   ...
}

queryParams is a String.

Now I want to cover the catch block. So in my test, I am writing like this:-

EasyMock.expect(anyObject(String.class).getBytes("UTF-8")).andThrow(new UnsupportedEncodingException());

But it doesn't seem to work. Can someone tell what can be here?