I'm assuming that SentVia and LblPrtInv are both controls on the subform.
In that case, your references to the fields are incorrect in the IF
statement.
1. Since FTimeBillingSub is a subform, Access won't find it with this
reference since it's not opened as a form itself but rather appears as a
control on a form (a subform control, but still a control and not a form).
Normally, you have to use a reference that refers to the main form and then
the subform control of the main form.
www.mvps.org/access has a very good
writeup on how to refer to one form from another, how to refer to subforms,
etc.
In this case, since you're trying to do this in the current event of the
subform itself, you can just use the Me. reference:
If IsNull(Me.SentVia) Then
2. You have to use a form-based reference to the label itself. Again, you
can use the Me. since you're trying to do this from within an event on the
subform
Me.LblPrtInv.Visible = False
The above two items are probably causing your problem
You might want to also check and make sure that the value of SentVia really
is null, and not just a zero length string (or for that matter, a non-zero
length blank string).
By the way, if you have this code, what are you going to do on new records
since the value of SentVia will be null? Your label will never show on new
records for entry of the field. Alternatively, if you have a default value
for SentVia, then you'll never have a null in there which means the label
will never be hidden.