Passing Null Value As Parameter In Update Stored Procedure

I am attempting to pass null as the input value from a series of
textboxes if the user does not input a value prior to submit. To try and do
this, I am using a vbscript function on this asp as follows:

CommentsAdd = IIf(Request.Form("Comments")="",NULL,Request.Form("Comments"))

I was hoping this would convert those emptystring textboxes to null.

Then, these variables are input in an update stored procedure called from
the same asp upon submit. This handles ntext and like fields in the table I
am updating, but a null value passed in this way for a numeric field chokes.
The error on the asp is something like, "unable to convert value of type
text to numeric". Can't null be passed as a parameter value when the field
type is numeric?

How does one pass null for a parameter's value in a stored procedure on an
asp? Am I missing something here?