Bristol-Myers Squibb and Celgene have a big problem on their hands. 

After the market closed Wednesday, Bristol-Myers’ largest investor, Wellington Management, moved to voice its opposition to its $74 billion mega-merger with Celgene. And analysts immediately flagged the move as a real threat to the deal, raising the risk that a major move like this could stir a widespread rebellion.

“As the No. 1 shareholder of BMY at about 8%, this is a real shareholder that introduces very real risk to the deal,” noted Baird’s Brian Skorney. And he counted off Wellington’s three reasons for objecting to the deal:

1) The transaction asks BMY shareholders to accept too much risk and the terms offer BMY shares to CELG shareholders at a price well below implied asset value.

2) Execution success could be more difficult to achieve than depicted by company management.

3) Alternative paths to create value for BMY shareholders could be more attractive.

Skorney continued: “While we believe the transaction appears beneficial to Bristol and will go through, this is not just a 1 million share position voicing discontent, and expect CELG to trade significantly lower tomorrow.”

The only other threat to this deal has come from the Celgene side, where Starboard Value’s activist chief has proposed a slate of 5 candidates for the board as it sniffs out opinions about the buyout among Celgene’s investors.

Bristol-Myers wasted no time in countering the move Wednesday, asserting that it got a real deal in buying the company at the right price. Their statement:

Since announcing the Celgene transaction on January 3, our Board and management team have had numerous conversations and meetings with our stockholders across our ownership base, including Wellington. We believe that we are acquiring Celgene at an attractive price, and that this transaction presents an important and unique opportunity to create sustainable value. We look forward to holding our special meeting of stockholders on April 12, 2019 and delivering the enhanced value this combination creates.