Invalid Column Name

I get a Invalid Column Name ' '. with this procedure. Can anyone see what migh be wrong?

Thanks,

SELECT A.CompanyName,C.FirstName,C.LastName,C.Client_ID,
CASE WHEN A.[CompanyName] IS NULL OR A.[CompanyName] = '' THEN C.[FirstName] +" "+ C.[LastName] ELSE A.[CompanyName] END AS DRName, C.Client_ID
FROM tblClients C INNER JOIN tblClientAddresses A ON C.Client_ID = A.Client_ID
WHERE (C.Client_ID = 15057) AND (A.MailTo=1) AND Convert(varchar(5), GETDATE(), 10) BETWEEN Convert(varchar(5), A.Startdate, 10) AND Convert(varchar(5), A.Enddate, 10) OR (A.Startdate Is Null) AND (A.EndDate Is Null)
GONormally, unless you've fiddled with your settings, SQL won't like double quotes... + " " +|||How should this be done?

Thanks,|||single quotes... + ' ' +