When I run FindBug on this code, it reports NO issues.

boolean _closed = false;

public void m1(@Nullable String text) {
    if(_closed)
        return;

    System.out.println(text.toLowerCase());
}

While here it finds issue as expected:

public void m1(@Nullable String text) {
    System.out.println(text.toLowerCase());  // FireBug: text must be nonnull but is marked as nullable
}

Why does it fail in first case?