err " this field name is not known " made me craze !!

hi there

when i try to preview the report on the crystal viewer :

first when i try to get all the data from the table it's work ok .. but when i try to made critirya with this code it's rase an err

Code:

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this field name is not known

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the code that used in critirya :

Code:

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report_for = "micro"
    report.RecordSelectionFormula = "{company_info.company_name}='" + report_for + "'"

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how to solve this case coz it's working for one time only and then the error rase > how come !!!

thanks