PHP Fatal error: Class 'WP_Ajax_Upgrader_Skin'


I try to install WP Smush Pro plugin and I got this error:

PHP Fatal error: Class ‘WP_Ajax_Upgrader_Skin’ not found in /data/sites/ on line 634, referer:

I’m using WordPress version 4.5. Unfortunately I can’t update the core because of one the plugins I’m using.

Can you please help me? :slight_smile:



  • Luís
    • Support

    Hi Granite ,

    Hope you’re doing well today!

    From what I searched in the support forum, this issue may be related to the outdated version of WordPress, because it was always solved after upgrade WordPress (at least to the 4.7 version).

    I will strongly advise to update WordPress, at least, because it’s a security risk.

    However, I am not sure if we can overcome this issue on your WordPress version, so, I would like to flag the SLS team (our code experts :slight_smile: ).

    To do this, can you please send us the FTP details of your website, so that the SLS can investigate further? Please send it privately through our contact form:

    Send in:

    Subject: “Attn: Luis Lopes”

    – WordPress admin username

    – WordPress admin password

    – Login url

    – FTP credentials (host/username/password)

    – Link back to this thread for reference

    – Any other relevant urls/information

    Cheers, Luís

  • Gerry
    • New Recruit

    I’m not the original poster but experienced this same issue when adding a WordPress site that has not been updated for so long.

    I was able to add the site by manually uploading and activating the WPMU Dev plugin but connections to the site were failing.

    Upgrading WordPress to the latest version fixed the issue for me.

  • Ash
    • WordPress Hacker

    There is no alternative I am afraid. We always try to be compatible with latest version of wordpress and try to maintain backward compatibility. But version 4.5 is way too old. There are lots of security fixes added in the lalter versions. So you must update the core to use Dev Dashboard plugin and make the connection.

    You can use smush plugin as a standalone plugin, but without the dashboard plugin you won’t be able to use the pro feature as well as you will miss the features of hub and automation.

  • Dominique
    • Recruit

    I get that you don’t support this version of WordPress anymore. My main point about this is mainly that you should implement an error message for this, so users know what’s going on instead of having to look in the PHP error logs. This would save a lot of time.

    This wasn’t noted in the opening post of this ticket.

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