WP Smush Pro - Image metadata not found

Hi,

We sometimes see this error message when trying to smush images: Image metadata not found. Here is a screen shot: https://www.evernote.com/shard/s134/sh/21bd276b-7a07-47bc-8a89-d08a4b2176b0/9f42757f04a2cbc79d414a8de7253fad

What does this error mean and how can we fix it?

Thank you!