Presumably you have other controls on the form such as the control button that cause the error. You could use an alternative approach e.g.
Code:
Private Sub cmdClear_Click()
Dim cControl As Control
For Each cControl In Me.Controls
Select Case TypeName(cControl)
Case Is = "TextBox", "ComboBox"
cControl = vbNullString
Case Else
End Select
Next
End Sub