As soon as Apple’s first foldable smartphone, the iPhone Duo, went on sale, some early buyers complained of cracks in its internal folding screen.

According to reports, some users’ phones’ screens were damaged near the folding part.

An Australian buyer claimed that after about six hours of use and opening the phone less than 20 times, he felt unusual resistance, after which the screen cracked and the display cracked along the folding line.

The user said that the phone had not been dropped and there was no significant damage on the outside. The Apple Store later replaced his phone.

Photos have also emerged from China and other countries that show bright lines and cracks between the folding screens. However, it is not yet clear how many phones have this problem and whether it is a widespread manufacturing or design defect.

Apple has also not yet released official findings on the cause of these incidents.