Sub-line of credit in case of facilities

Hi,

I have created various lines of credit in TCL1 such as

1.Funded

2.Non-Funded, etc

I created a facility using 56B. At the structure tab, there is an option 'Other lines'. Here, I want to have the Prim.line of credit as the Main line of credit and the sub-lines of credit as

Funded and Non-funded.

However when I try to do that I get an error FTR_FC012:Main Line of credit cannot be selected.

Please guide.