VBA not handling end of For...Next when inserting into SQL

Hi Experts -

I have a spreadsheet that is looking at a concatenated list of values in column c and inserting them into a SQL table.  It's working great until it hits the first empty cell in C.  It's trying to send a null value to the table, so I'm getting this error:  
Here's the code:

Private Sub CommandButton1_Click()
Dim rng As Range
Dim cell As Range
Dim con As Object
'connect and clear existing data in table
Set con = CreateObject("adodb.connection")
    con.connectionstring = "Provider=MSDASQL.1;Persist Security Info=False;User ID=sa;Data Source=BTS;Initial Catalog=vmfgtest; password=blocked"
    sSQL = "delete osk_release_num1"
    con.Open
    con.Execute sSQL

'loop through data in column c and insert into table
Set rng = Worksheets("sheet1").Range("c2:c170")
For Each cell In rng
    Set con = CreateObject("adodb.connection")
    con.connectionstring = "Provider=MSDASQL.1;Persist Security Info=False;User ID=sa;Data Source=BTS;Initial Catalog=vmfgtest; password=blocked"
    sSQL = "insert into osk_release_num1 (release_num, part_num, due_date) values ( " & cell & " )"
    con.Open
    con.Execute sSQL
 Next cell
 
 a = MsgBox("Success!", vbOKOnly)
 End Sub

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I tried using Do Until IsEmpty with a loop, that was even worse.

I just am looking as to how to make this end neatly and produce the "Success" message box.   Thanks so much!!