The Parameter Is Incorrect

I am getting an error the parameter is incorrect when using a parameter within a case statement. Here is my SQL:
SELECT J.Commitment, J.Vendor, A.Name, J.Category, J.Transaction_Type, J.Job, CASE WHEN J.Transaction_Type = 'AP cost' AND
J.Accounting_Date < @MonthEndDate THEN J.Amount END AS InvoicedtoDate, C.Date, C.Delivery_Date
FROM JCT_CURRENT__TRANSACTION AS J INNER JOIN
JCM_MASTER__COMMITMENT AS C ON J.DBID = C.DBID AND J.Commitment = C.Commitment LEFT OUTER JOIN
APM_MASTER__VENDOR AS A ON J.DBID = A.DBID AND J.Vendor = A.Vendor
WHERE (J.Commitment <> ' ') AND (J.Job <> ' ')
ORDER BY J.Job, J.Commitment
If I replace @MonthEndDate with a date value it will run but as soon as I run the SQL with the parameter it gives me the error the parameter is incorrect. @MonthEndDate is data type datetime.
Thanks for your help.