Creating new SpannableStringBuilder returns null

Here is my unit test. Somehow when I create instantiate SpannableStringBuilder instance with a non-null string argument, it appears to be null. What's wrong with this code?

public class CardNumberTextWatcherTest {

private CardNumberTextWatcher watcher;

@Before
public void setUp() throws Exception {
    watcher = new CardNumberTextWatcher();
}


@Test
public void afterTextChanged_cardNumberDividedWithSpaces() {
    assertTransformText("1111222233334444", "1111 2222 3333 4444");
}

private void assertTransformText(final String testStr,
                                 final String expectedStr) {
    final CardNumberTextWatcher watcher = new CardNumberTextWatcher();
    final Editable actualStr = new SpannableStringBuilder(testStr);
    // When I debug I see that actualStr value is null. Therefore test 
    //doesn't pass. What's the problem?
    watcher.transformText(actualStr);
    assertEquals(expectedStr, actualStr.toString());
}


}

The problem was in the testing directory. As soon as SpannableStringBuilder is a part of Android SDK, it couldn't be resolved in regular JUnit test directory. When I moved my class to androidTest directory, everything started to work as expected.