Get Date Parameters for Query from OINV

I want to prompt the user to select dates from the existing dates of the OINV file. I have tried this but get an error:

DECLARE @FromDate datetime, @ToDate datetime

SET @FromDate = (SELECT Min(DocDate) FROM OINV WHERE DocDate = '[%0]') 

SET @ToDate = (SELECT Min(DocDate) FROM OINV WHERE DocDate = '[%1]')

Any suggestions?