Error During Validation Conditional Split

Hi,

In my package i'm using conditional split operator to do some validation. The source is a flat file and the date field is of the data type DT_STR. The expression i'm using for validation is

ISNULL(DtCustPay) || DtCustPay < 1

But i'm getting the following error:

TITLE: Microsoft Visual Studio
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Error at Data Flow Task [Validation [366]]: The data types "DT_WSTR" and "DT_I4" are incompatible for binary operator "<". The operand types could not be implicitly cast into compatible types for the operation. To perform this operation, one or both operands need to be explicitly cast with a cast operator.

Error at Data Flow Task [Validation [366]]: Attempt to set the result type of binary operation "DtCustPay < 1" failed with error code 0xC0047080.

Error at Data Flow Task [Validation [366]]: Computing the expression "ISNULL(DtCustPay) || DtCustPay < 1" failed with error code 0xC0047084. The expression may have errors, such as divide by zero, that cannot be detected at parse time, or there may be an out-of-memory error.

Error at Data Flow Task [Validation [366]]: The expression "ISNULL(DtCustPay) || DtCustPay < 1" on "output "Invalid Pay Date" (453)" is not valid.

Error at Data Flow Task [Validation [366]]: Failed to set property "Expression" on "output "Invalid Pay Date" (453)".

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ADDITIONAL INFORMATION:

Exception from HRESULT: 0xC0204006 (Microsoft.SqlServer.DTSPipelineWrap)

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BUTTONS:

OK
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Help needed please.

Thanks