The ongoing merger talks between T-Mobile and Sprint are reportedly drawing criticism from some T-Mobile shareholders.

CNBC, citing unnamed sources, reports that although the discussions continue to progress, the shareholders in question are concerned about the cost of a deal between the nation’s third- and fourth-largest wireless carriers.

Reports previously indicated that the companies are considering a stock-for-stock exchange that would give T-Mobile parent Deutsche Telecom control of a combined company. Under that proposal, the deal would not place a premium on the value of Sprint stock, but some industry analysts content that Sprint is currently overvalued.

The larger hurdle to a merger, however, could be U.S. antitrust regulators, who must consider whether a merger between two of the four large wireless providers would stifle competition in the market.