Hidden fields/visible false fields and inserting values

[Chumley the Walrus]

In an data insert script, I'm trying to insert a field (userid) with a
textbox object, but I want it to be invisible to the user so the user
cannot change it inside the textbox:

<td align="right">ID- don't change!</td>
<td> <asp:textbox id="frmuserid" runat="server" value="10"
visible="false"/>
</td>

....I don't get an error when inserting this, but the 'userid' does not
get inserted into the database. Wondering what's missing. In old asp,
an input type=hidden worked fine, as you can assign it a hardcoded
value inside the tag.
When trying to use the old school input type= hidden name=frmuserid
value=10, it does insert into the database, but I get a comma (from
the form post) in front of the 10, which renders it unreadable when
trying to use it afterwards.

thanks
chum