Adding Customfield Value to a Process through PSI

Hello,
i declared my Customfield as a Process so that i can make it visible via the WebApp.
The Problem is that i get the following Error if i am trying to Update through
QueueUpdateProject

"ProjectServerError(s) LastError=CustomFieldInvalidUID Instructions: Passthis into PSClientError constructor to access all error Information"

If my CustomField is declared as a Project it runs without Error.
Here is my Code.

ProjectDataSet projectList = projectSvc.ReadProjectList()...

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