Stacks error

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I'm running 5.6.3.2 and recently went into edit my stacks. I got the following error message:

Fatal error: Call to a member function getCollectionID() on a non-object in /home/sites/churchinthepeak.org.uk/public_html/updates/concrete5.6.3.2/concrete/core/models/collection_version.php on line 46

I don't usually use stacks so don't know how long the error might have been there.

Can anyone help please?

Justyn