Phan complains because $user_id can potentially be null, but addWhereFld doesn't take null, so phan would theoretically be right here. However, as you noticed, it cannot infer from the if above that the null case is discarded. This is because phan doesn't know that calling dieWithError() will throw an exception, and this, in turn, is a constant source of pain, see T240141. I suggest suppressing the issue inline, and mentioning that task in the suppression message so it can be easily identified. In short: @phan-suppress-next-line PhanTypeMismatchArgumentNullable T240141.