insert into

I have an insert into query which is as follows;

strgoaldescription = InputBox("Type new goal.")
strservicetype = InputBox("This goal is for what service?")
intprGroup = InputBox("Indicate the pr_group number if any")
intprGroup = IIf(intprGroup = "", Null, intprGroup)
strsql = "insert into goal (goal_description,los_id,pr_group_id) values('"
& strgoaldescription & "', '" & strservicetype & "', " & IIf(intprGroup = "",
Null, intprGroup) & ")"

the query throws an error when intprgroup is null. There will be times when
this is the case so how do I pass null to the field via an insert into query.
the field allows nulls to be in it.