SQL & PL/SQL

iam having this function in SQL Server 2005 it is forking fine in SQL Server 2005 but running this on oracle 11g gives error this function is returning the name of employee if his user_id is passed

create or replace
FUNCTION fn_UserEmployeeFullName----------- this name of function
(
v_UserID IN NUMBER---------------this value iam passing to a function
)
RETURN NVARCHAR2
AS
v_EmpName NVARCHAR2(150);--------------this user Declared local variable
BEGIN
SELECT CASE NVL(Employee_Mst.FirstName, '')--------------firstname which is a column in table
WHEN ''
THEN ''
ELSE Employee_Mst.FirstName || ' '
END || CASE NVL(Employee_Mst.MiddleName, '')--------------MiddleName which is a column in table
WHEN ''
THEN ''
ELSE Employee_Mst.MiddleName || ' '
END || CASE NVL(Employee_Mst.LastName, '')--------------.LastName which is a column in table
WHEN ''
THEN ''
ELSE Employee_Mst.LastName || ' '
END
INTO v_EmpName
FROM User_Mst
JOIN Employee_Mst
ON User_Mst.EmpId = Employee_Mst.EmpID
WHERE User_Mst.UserID = v_UserID;

RETURN v_EmpName;---------------------- now returning this value

END;

this gives me compilation error

Error(11,19): PL/SQL: ORA-12704: character set mismatch

iam not understanding plz tell me what is this : character set mismatch