Setting Dynamic Range

Hi, I'm building a form that contains a combo box where the range will change. This is the relevant part of my code that is in the form

Dim MyRange as Range
           Dim LstRow as Long  

LstRow = Sheets("Sheet1").Range("A1").End(xlDown).Row ' This works fine

With Sheets("Sheet1")
MyRange = .Range("A1:A" & LstRow)
End With

Whenever I run this I get an object variable error. Would be grateful for any assistance or pointers

Many thanks