Runtime Error 6028

I'm adding a UserForm to a Word template. Upon submitting the form, I receive 'Runtime Error 6028 - The range cannot be deleted'. The debugger points to the following script:

Private Sub CmdSubmit_Click()
Application.ScreenUpdating = False
With ActiveDocument
--> .Bookmarks("Text1").Range.Text = txtUserName.Value
.Bookmarks("Text2").Range.Text = txtTest1.Value
.Bookmarks("Text3").Range.Text = txtTest2.Value
.Bookmarks("Text4").Range.Text = txtTest1.Value
End With...

Runtime Error 6028