Dropdown List in nested MasterPage creates an error

[Guest]

All I did was change this mockup page to really work by adding a dropdown
and I get this error:

Control 'ctl00_ContentPlaceHolder1_cmbSL' of type 'DropDownList' must be
placed inside a form tag with runat=server.

do I need to change code in the master page or in the ContentPlaceHolder?

 

[Aidy]

The error is pretty self-explanatory. Your page will have a form tag at the
top (be it at the master page level or sub page) and asp controls need to be
inside that form.