VBA in query error evaluating false part of IIF - vba

Question

I have the following column in a query.
iif(Len([Field1])=0,0,Asc(Mid([Field1] & "",Len([Field1]))))
The idea is that it should return the ASCII value of the last character in a string field.
The problem is that if Field1 is blank the statement errors with the following message: "Invalid procedure call or argument (Error 5)". If the field is blank it should return 0.

Answer 1

Assuming blank means either Null or an empty string, you can concatenate an empty string to [Field1], and if the combined length is 0, return 0.
The Right() function is more direct than Mid() to get the last character.
SELECT IIf(Len([Field1] & "")=0,0,Asc(Right([Field1],1)))
FROM YourTable;

Answer 2

By all means you have HansUp's answer to go ahead. :)
..At any point of this querying voyage, your query with IIF gets very very long... ;) here a head start with a UDfunction that uses REGEX. In your case the most important aspect is to validate the input - not much of getting the ASCII value. Since your concern mainly on last character, what if your Field1 contains alphanumeric value and if your last character happens to be digit, special character instead of String. Well, just to be safe, we could validate that in the following funciton which will respond only to Strings. Another advantage is that you can re-use this function within your db.
Perhaps this really is complicating your process :D
Option Compare Database
Option Explicit
Public Function GetASCII(strField1 As String) As Integer
Dim regex As Object
Dim strTemp As String
Set regex = CreateObject("vbscript.regexp")
'--assume strField1 = "hola", you may enter different types
'-- of values to test e.g. "hola1, hola$ , hola;
With regex
.IgnoreCase = True
.Global = True
.Pattern = "[a-zA-Z]+"
'--check for null
Select Case IsNull(strField1) '-- validates any other datatype than String
Case True
GetASCII = 0
Case Else
Select Case IsError(strField1)
Case True
GetASCII = 0
Case Else
Select Case IsEmpty(strField1)
Case True
GetASCII = 0
'--check if entire string is String
'--only (no special characters, digits)
'--you may change this to only check the
'----last character if your Field1 is alphanumeric
Case Else
Select Case .Test(strField1)
Case True
strTemp = Mid(strField1, Len(strField1), 1)
GetASCII = Asc(strTemp)
Case Else
GetASCII = 0
End Select
End Select
End Select
End Select
End With
Set regex = Nothing
End Function
Note: Thought this would be helpful in the long run :) An Access Query Primer Article.