MapBasic issue in creating Thematic with Shade

I’m trying to write a bit of code that looks the columns of a table and if they match by text, then generate a thematic of predefined ranges.

So far:

Include "MapBasic.def"

Declare Sub Main

Sub Main()
    Dim ColumnCount as integer
    Dim ColumnName as string
    Dim TableName as string
    Dim aColAlias as Alias
    Dim WinID as integer

    WinID = FrontWindow()
    
    TableName = tableinfo(1,TAB_INFO_NAME)
    
    'Assume table 1 for this test
    for ColumnCount=1 to tableinfo(1,TAB_INFO_NCOLS)

        ColumnName =  ColumnInfo(1,"COL" & ColumnCount,COL_INFO_NAME)
        
        if instr(1,ColumnName,"RSRQ") >1  and instr(1,ColumnName,"SS") > 1  then
            print TableName & "." & ColumnName 'works as expected
            aColAlias = ColumnName
                    
            'Create Thematic
            Shade Window WinID TableName with aColAlias
            ignore 0
            Ranges
            -1 : -4 Symbol(108,6750207,10,"Wingdings",0,0),
            -4 : -7 Symbol(108,8454016,10,"Wingdings",0,0),
            -7 : -10 Symbol(108, YELLOW,10,"Wingdings",0,0),
            -10 : -13 Symbol(108,16757529,10,"Wingdings",0,0),
            -13 : -60 Symbol(108,RED,10,"Wingdings",0,0)
            
            exit For
        End If
    Next

End Sub

The Shade command always fails with ‘Invalid variable Type’

I’ve tried replacing the Column Alias with the string of the column name, but that doesn’t work either. I’m sure its something simple, but I just can’t see it.