Not even Apple knows exactly how many people are reading its months-old News app, according to a recent report.

Apple has been underestimating the amount of readers using the app since it launched last September, The Wall Street Journal reported Sunday. The News app aggregates articles from top media outlets and allows readers to view them inside the app, rather than the publications' mobile websites.

Read MoreApple is in a 'disastrous' situation: Trader

The erroneous numbers have been passed along to Apple's 100-plus global publishing partners, according to the Journal.

While publishers don't pay to post content to the app, the underreported readership numbers may have hurt partners' ability to sell advertising and set the correct ad rates, the paper reported.

Publishers keep 100 percent of revenue when they sell their own ads on the app. They can keep 70 percent of ad revenue if Apple sells ads on their behalf, according to the Journal.

Read MoreInside Yahoo's troubled advertising business

The publication reports that Eddy Cue, Apple's senior vice president of Internet software and services, said that the tech giant is working on fixing the problem.

Apple did not immediately respond to CNBC's request for comment.

Read the full Wall Street Journal report here.