SQL: Set variable to a cell value inside a stored procedure

I keep getting a MSG 201:

Procedure or function 'spBookReturn' expects parameter '@bookID', which was not supplied.


Every time I run this a sp where I try to define a variable as another cell value:

CREATE PROC spBookReturn
@loanID UNIQUEIDENTIFIER,
@bookID UNIQUEIDENTIFIER OUTPUT,
@custID UNIQUEIDENTIFIER OUTPUT
AS
BEGIN
BEGIN TRANSACTION tBookReturn
UPDATE BorrowedMaterial SET returned = 1 WHERE loanID = @loanID;
UPDATE BorrowedMaterial SET returnedDate = GETDATE() WHERE loanID = @loanID;
SET @bookID = (SELECT TOP 1 bookID FROM BorrowedMaterial WHERE loanID = @loanID);
UPDATE Books SET nHome = nHome + 1 WHERE ID = @bookID;
COMMIT TRANSACTION tBookReturn;
END;

EXEC spBookReturn '546A444A-3D8D-412E-876D-2053D575B54F'


Does anyone know why the way I have defined the @bookID variable doesn't work and how I can make it work?

Thanks.

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