Unable to specify .RowSource for Combo Box

I am trying to make a dependent combo box in Excel using VBA. Here is my code:

Option Explicit

Private Sub conduitTypeBox_Change()

Select Case conduitTypeBox.Value
    Case Is = "IMC"
        conduitSizeBox.RowSource = "imcSize"
    Case Is = "LFMC"
        conduitSizeBox.RowSource = "lfmcSize"
    Case Is = "RMC"
        conduitSizeBox.RowSource = "rmcSize"
    Case Is = "PVC (Schedule 80)"
        conduitSizeBox.RowSource = "pvc80Size"
    Case Is = "PVC (Schedule 40)"
        conduitSizeBox.RowSource = "pvc40Size"
    Case Is = "PVC (Type A)"
        conduitSizeBox.RowSource = "pvcaSize"
    Case Is = "PVC (Type EB)"
        conduitSizeBox.RowSource = "pvcebSize"
End Select

End Sub

The concept is that the user selects a type of conduit (ex. "IMC") in the first combo box and then a second combo box populates with sizes unique to the conduit type. I created a name for the range of the data using the Name Manager (and ensured it has the scope of the entire workbook), however I am getting run-time error '438': Object doesn't support this property or method. I understand what the issue is, but I'm not entirely sure why it's an issue. When I type "conduitSizeBox.", RowSource does not appear in the list of available properties. Would anyone have any idea why?

I tried looking for other properties and didn't find anything that fit my needs.

Use the Listfillrange property instead of Rowsource.