invalid 3d element-child of corridor

I have created a corridor and now wish to have the template follow some linework that is not at a default distance that is coming from the template. in the template, in the point menu I have added the feature definition I wish to follow. when I try to add the linework as an external reference I get the following message "invalid 3d element-child of corridor". the linework is in a separate file with the correct feature definition and that file is 2d. do not understand the message. thanks for your help, brian