The DUP named three issues with Boris Johnson's proposals - customs, consent issues and VAT. But what exactly are their worries?
Customs
The DUP is digging in over the prospect of a customs border between Northern Ireland and the rest of the UK.
The new back-up arrangements that would kick in under Mr Johnson's proposals would effectively see Northern Ireland following EU rules much more closely than the rest of the UK.
Consent
One of the key things that the PM has proposed is a way of giving Northern Ireland communities a say on whether or not to continue those arrangements.
But what the DUP has an issue with is that the mechanism that has been negotiated would give the suspended Stormont Assembly a final vote on whether to keep these rules going.
The DUP's concern is that might then mean that nationalists could keep Northern Ireland more tied to the EU than they would want, indefinitely.
VAT
Another major issue in the PM's proposals is whether or not EU VAT rates would apply in Northern Ireland - something the DUP says remains unclear, despite hours of talks with the government over the last few days.