calling functions in IN operators

Question

I wrote a function (dbo.fFunctionCols) with 2 parameters (1st is tablename, 2nd is search pattern for column names) that returns certain column names from a table and passes them to the query below in which the same table is unpivoted. The query should end up as a view.

SELECT Id, AB, vTime, vItem, vCount
FROM
   (SELECT Id, AB, vTime,
    dbo.fFunctionCols('tablename', 'abc%def')
    FROM dbo.[tablename]) AS piv
UNPIVOT
   (vCount FOR vITEM IN
      (SELECT dbo.fFunctionCols('tablename', 'abc%def') AS t1
   ) AS unpiv;
GO

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SQL Server always returns:

Msg 156, Level 15, State 1, Line 8
Incorrect syntax near the keyword 'SELECT'.
Msg 102, Level 15, State 1, Line 8
Incorrect syntax near ')'.

The statement: SELECT dbo.fFunctionCols('tablename', 'abc%def') AS t1 works properly if executed. It delivers the correct list of the relevant column names seperated with commas and enclosed in square brackets.

What´s the error? Why can the function not be used inside the IN operator?