Error using Alias component annotation

I get this error when I try to use the Alias annotation on a

You have attempted to dereference a scalar variable of type class
java.lang.String as a structure with members.

The error occurred in E:\web\sites\cf9.local\webapps\ROOT\coldbox
\coldbox-platform\system\ioc\config\Mapping.cfc: line 506

It's the mappings variable that is throwing the error.