What is the error undefined reference to `vtable for classname'

Question

I am having a hard time understanding why I get the error undefined reference to `vtable for Item'

├── build_debug
│   └── src
│       ├── CMakeFiles
│       └── Makefile
├── build_release
├── CMakeLists.txt
└── src
    ├── CMakeLists.txt
    ├── Item.cc
    ├── Item.h
    ├── main.cc
    └── Makefile

contents of item.h

#ifndef ITEM_H
#define ITEM_H

class Item
{
public:
    Item();
    virtual ~Item();
};
#endif

contents of item.cc

#include "item.h"
Item::Item(){}

And the contents of CMakeLists.txt

add_compile_options(-fpermissive)

set(SOURCES 
    main.cc
    item.h
    item.cc)

message (${SOURCES})
add_executable(app ${SOURCES})