Identify why Django `F` fields are broken

Using Django's `F` fields to optimise the SQL queries made in a `Model.aupdate()` operation seems to result in the following error: `'strikes': ["F(strikes) - Value(1)" value must be an integer for both places.]`. This has resulted in the need to use an update to the field's value in Python, along with a call to `Model.asave()` which is less efficient & performant.