Verify Fields Are Not Null

[jo15765]

I have a form with 2 fields ([field1],[field2]), and on this form there is a button that when pressed will run an update query based off the values of these fields. BEFORE the update query executes, how can I 1st check that [field1] is not null, then check [field2] is not null? Some pseudocode of what I want
check [field1]
Is null - error message "Field 1 is null"
Is not null check [field2]
Is null - error message "Field 2 is null"
Is not null - run update query

[June7]

If IsNull(Me!field1) Or IsNull(Me!field2) Then
MsgBox "Must enter data into both Field1 and Field2"
Else
'run UPDATE query
End If