Since it's a function, it looks like you are missing something.
See the details about the error here: http://www.sql-server-performance.com/2007/parameters-not-supplied-is/
This error message appears when you don’t supply a value expected by a user-defined function.Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.Resolution:
Error of the Severity Level 16 are generated by the user and can be fixed by the SQL Server user. You must supply values for each parameter in a user-defind function as long as there is not default value for that parameter defined. These supplied values must explicitly or implicitly be convertible to the parameter in question.