Error with variable substitution..

Hi

I am getting following error message for variable substitution

*Message processing failed. Cause: com.sap.aii.af.ra.ms.api.RecoverableException: Error during variable substitution: com.sap.aii.adapter.file.varsubst.VariableDataSourceException: The following variable was not found in the message payload: date_stamp: com.sap.aii.adapter.file.configuration.DynamicConfigurationException: Error during variable substitution: com.sap.aii.adapter.file.varsubst.VariableDataSourceException: The following variable was not found in the message payload: date_stamp

<?xml version="1.0" encoding="UTF-8" ?>

- <I813_EModMCS_IB_MT>

- <Records>

<ModMod>111</ModMod>

<DesModdcr>222</DesModdcr>

- <Date_Stamp>

<Date_Stamp>20080338</Date_Stamp>

</Date_Stamp>

</Records>

</I813_EModMCS_IB_MT>

date_stamp payload:I813_EModMCS_IB_MT,1,Records,2,Date_Stamp,3,Date_Stamp,1

Would appriciate if you can correct the variable substitution...

Regards