Crystal Reports If IsNull

Message

   I get A String is Required here with this formula - can anyone help me identify my problem??

//This formula checks to see if DRG expected reimbursement is empty, possibly indicating a Medicaid patient
if IsNull ({HSP_ACCOUNT.DRG_EXPECTED_REIMB}) then "Medicaid"
//If the DRG expected reimbursement is provided, display the value stored in the DRG Expected Reimbursement field
else {HSP_ACCOUNT.DRG_EXPECTED_REIMB}

Thanks!! 