Amazon is restricting merchant access to its warehouses during the holiday shopping season. Capacity issues that have created problems in the past and the online retailer is taking steps to avoid it this year.

Bloomberg reports that Amazon is freezing new merchants out of its fast-delivery service Fulfillment By Amazon until Dec. 19, preventing them from sending inventory to its fulfillment centers in advance of the holidays, said the people, who asked not to be identified because they weren’t authorized to speak about the issue.

Sending inventory to Amazon warehouses is a critical step for products to be eligible for the two-day delivery service coveted by Amazon’s most loyal shoppers, those paying $99 a year for Prime memberships. It’s is a big boost to sellers who will see an increased exposure, compared to shipping products on their own, directly to customers.

Amazon usually has cut-off times for merchants in select categories — such as toys — to get inventory to warehouses in advance of the holidays. This year, however, for the first time the restrictions on new sellers apply to all categories of items and came without advance warning, the people said. Amazon declined to comment.

Earlier in the year Amazon changed its fees to penalize merchants who store slow-moving or off-season merchandise in its warehouses during the holidays and reward merchants who send in fast-moving products such as hot-selling Christmas toys. Amazon makes more money from merchants who pay the company in exchange for access to its more than 300 million online customers.

Amazon went on a building spree this year to be better prepared for the holiday rush, opening 21 new warehouses in the first nine months of this year, compared with 10 in the same period of 2015.