Using the workflow variables schema in a chain generates a warning

Say I have a workflow MyWF. It has a priority variable.

In a chain not run in the context of a workflow (so, we can't use @{WorkflowVariables["priority"]} for example), I load the workflow document (I know its Id) and store the priority:

Fetch > Document
    value: the hid
Execution Context > Set Context Variable
    name: the_priority
    value: @{Document["var_MyWF:priority"]}

This generates the following warning in the chain:

The value of parameter 'value' from operation Context.SetVar may be incorrect

It should not display the alerts since var_MyWF schema exists with the MyWF workflow.