How to fix this in crystal report "a string is required here! "?

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VB

CrystalReports

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I have a column in tblRegPatient name Date and its DataType is "Date".
and I am trying to filter the data in crystal report by Date, while writting the formula of

If {tblRegPatient.Date} >= {?start_date} and {tblRegPatient.Date} <={?end_date} then true

there is error saying "A string is required here".
The type of start_date and end_date is Date but when I Browse Data of column Date in Field Explorer it is "String" but its datatype is "Date" in my tblRegPatient.. It is strange.. Please help me. Thanks!

What I have tried:

If {tblRegPatient.Date} >= {?start_date} and {tblRegPatient.Date} <={?end_date} then true