Subtring error when querying

I am getitng a substring error when querying with this string, when I reverse things out it comes down to the middle name is sometimes not there, how can I account for this when middle name is null thanks

Select      p.strSSN,
      substring(s.sidstrName_IND, 1, charindex(' ', s.sidstrName_IND, 1)-1) Last,
      substring(s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)-charindex(' ', s.sidstrName_IND, 1)-1) First,
      substring(s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)+1, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)+1)-charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)-1) Middle,
      case charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)+1) when 0 then '' else
      substring(s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)+1)+1, len(s.sidstrName_IND) - charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, charindex(' ', s.sidstrName_IND, 1)+1)+1)+1) end Suffix,
      s.sidstrST_ADDR,
      s.sidstrADDR_CITY,
      s.sidstrSTATES_US,
      s.sidstrZIP_CODE,
        Sum(CONVERT(decimal(18,2), REPLACE(p.strBasePay, '$', ''))) + SUM(Convert(Decimal(18,2), Replace(p.strDiffPay, '$', ''))) as TotalPayTaxable,
      SUM(Convert(Decimal(18,2), Replace(p.strTTWH, '$', ''))) as TaxWithHeld
From      tblsidpers as s INNER JOIN
      tblSadPay as p on p.strSSN = s.sidstrSSN_SM
where      dtPayPaid >= '8/01/2008' and dtPayPaid <= '8/30/2008'
Group by      s.sidstrNAME_IND, p.strSSN, s.sidstrST_ADDR, s.sidstrADDR_CITY, s.sidstrSTATES_US, s.sidstrZIP_CODE Start Free Trial