Execute SQL Not Working

All - 

I have an execute sql task set up as follows:

ResultSet = Single Row

Connection Type = OLE DB

SQL Source Type = Direct Input

SQL Statement = execute dbo.StoredProcedure @FileName = ? @FileCount = ? OUTPUT

(The sp queries a table with @FileName to see if the supplied file name already exists, and outputs 1 if true and 0 if false)

Parameter Mapping:

Variable Name = User::FileName, Input, Long, 0, -1

Variable Name = User::FileCount, Output, Long, 1, -1

It is returning the following error: "Executing the following query...failed with the following error: "Invalid Parameter Number." Possible failure reasons : problems with the query, "ResultSet" not set properly...etc

Any ideas?

Thank you!

Bonediggler