Does not support EnsuresNonNull

I suppose NullAway does not perform complex data flow analysis now, so cannot handle the following example. Besides suppressing this warning, how can we handle this in NullAway?

public class CharSequenceReader {

private int pos;
@Nullable private CharSequence seq;

@EnsuresNonNull("this.seq")
private void checkOpen() throws IOException {
if (seq == null) {
throw new IOException("Not open.");
}
}

public int read() throws IOException {
checkOpen();
return seq.charAt(pos++); // NullAway warns here.
}

}