Why did PM not get legal advice?

I put in a Freedom of Information request to the Prime Minister’s Office on 14th December 2018 as follows:

“The Prime Minister has failed to publish the full legal advice relating to the Withdrawal Agreement despite being required to by the House of Commons in a historic resolution passed last week. The Prime Minister has published legal advice on the Northern Ireland border only.

Bill Cash MP has repeatedly asked the Prime Minister whether the law officers were consulted as required under the Ministerial Code in good time before Chequers and the Withdrawal Agreement. Many fundamental legal questions remain unanswered and unresolved.

May I please have a copy of all legal advice given to the government relating to the draft Withdrawal Agreement.”

I received the following response on 9th January 2018.

“I am writing to advise you that following a search of our paper and electronic records, I have established that the information you requested is not held by the Cabinet Office.”

This response implies that the Prime Minister has not received any such legal advice. This is an astonishing revelation and I find it hard to believe. I find it inconceivable that a UK Prime Minister would negotiate such an important and historic agreement without seeking and receiving legal guidance.

Is this response a deliberate lie; a crude attempt to avoid disclosure of the information ahead of the parliamentary vote on the draft Withdrawal Agreement?