Cannot add designer form classes through wizard to certain paths in a subdirs project

To reproduce:

• Open "build/Qt/build.pro" project
• Right-click on SomeLib project
• Select 'Add new...'
• Select 'Qt Designer Form Class'
• Next
• Select 'Browse'
• Point it to "../../src/" (<project>/externals/SomeLib/src/)
• Ok
• Open
• Next

"Add to project" does not contain the project. Pressing Finish causes files to be created and the designer opened, but they are not added to the project.