Error running validation code

Hi All,

I'm running a code where I create 4 different dropdown lists in a range of cells. However, on one of the lines I get an error "Application-defined or object-defined error." The code looks exactly the same as the 3 lines above it just with different cells it's being assigned to. Any advice would be appreciated. Here's my code:

Worksheets("Main").Range("C16,D13,D17,E13,E18,F14,F15,F16,F17").Validation.Add 3, , , Join(Application.Transpose(Sheets("DropDown").Columns(12).SpecialCells(2)), ",")
                Worksheets("Main").Range("C14,C15,D14,D15,D16,E14,E16,E17").Validation.Add 3, , , Join(Application.Transpose(Sheets("DropDown").Columns(2).SpecialCells(2)), ",")
                Worksheets("Main").Range("C17,D12,D18,E12,E19,G14,G15,G16,G17").Validation.Add 3, , , Join(Application.Transpose(Sheets("DropDown").Columns(32).SpecialCells(2)), ",")
                Worksheets("Main").Range("C18,D11,D19,E11,E20,H14,H15,H16,H17").Validation.Add 3, , , Join(Application.Transpose(Sheets("DropDown").Columns(37).SpecialCells(2)), ",")

Thanks in advance