McDonald's is under fire from customers accusing the burger chain of serving mozzarella sticks that are missing a key ingredient: cheese.

Customers are posting pictures of hollowed-out mozzarella sticks online and accusing the burger chain of serving them "fried air."

McDonald's launched mozzarella sticks nationwide this month as part of a new "McPick 2" promotion, which allows customers to pick two of the following for $2: a McDouble, a McChicken, small fries, and mozzarella sticks.

The mozzarella sticks are also available on their own, costing $1 for three sticks.

But photos posted online seem to show that for many customers, all three of the mozzarella sticks in their orders were missing a cheese filling.

When we tried some mozzarella sticks in November, all three sticks in our order contained cheese.

McDonald's did not immediately respond for comment.

Here's what people are saying on Twitter:

—Jonathan Lee Dai (@JoeNaeThin) January 27, 2016

—El Perro Grande (@Tha_Real_Drizzy) January 9, 2016