Invalid Use Of Null

Why can I not insert a null value at : rec("Outlet_ID") = hyphenit(rec("Outlet_ID")) in the code.

It throws a run-time error 94. invalid use of null.

the table has allow zero length strings set to yes.

Code: 'If RS.EOF then we're at the end of the recordset already, i.e. no records If rec.EOF Then rec.Close Set rec = Nothing Set dbs = Nothing Else rec.Requery rec.MoveFirst While Not rec.EOF rec.Edit rec("Outlet_ID") = hyphenit(rec("Outlet_ID")) rec.Update rec.MoveNext Wend End If

hyphenit:
Code:Function hyphenit(str1 As String) 'check for hyphen already present using instr If IsNull(str1) Then hyphenit = "" Exit Function ElseIf (InStr(1, str1, "-") <> 0) Then hyphenit = str1 Exit Function Else 'if no hyphen, then iterate thru the characters one at a time until isnumeric = true 'then split the string using left(str1, character_pos -1) and mid(str1, character_pos) 'and concatenate with hyphen Dim character_pos As Integer character_pos = 1 For character_pos = 1 To Len(str1) If IsNumeric(Mid(str1, character_pos, 1)) = True Then hyphenit = Left(str1, character_pos - 1) & "-" & Mid(str1, character_pos) Exit Function End If Next 'if isnumeric is NOT true and end of string is reached then hyphenit = str1 End If 'no hyphen found, nor any numbers hyphenit = str1End Function

thanks