Validate List - Restriction

I have a sheet that contains two values in seperate columns that I need to concatinate into one value for the drop down. If I use one value my code works but doesn't show all the values. If I use both values I get a 1004 - Application Defined or object defined error. I bolded the section that if I add I get the 1004 error. Is there some sort of restriction when creating the validate list for a drop down? My list is not reaching 65000 characters which is the string cut off.

Dim intAdEventCol As Integer, intAdEventNmCol As Integer, intRow As Integer
Dim intLastRow As Integer, intLastCol As Integer
Dim strAdEvent As String, strAdEventNm As String
Dim strAllAdEvent As String

Do Until intRow > intLastRow
strAdEvent = Worksheets(gstrcDataWorkSheet).Cells(intRow, intAdEventCol).Value
strAdEventNm = Worksheets(gstrcDataWorkSheet).Cells(intRow, intAdEventNmCol).Value
If intRow = 2 Then..........................