Steps To reproduce:
1. Go to control panel
2. Click on Documents and Media
3. Click manage > document types
4. Click Add
5. Type in 'Test' into the name box and add a text field in the main metadata fields and then click save
6. Close the document types window
7. Click Add > Test
8. For file, choose any file
9. Input a title ('test') and input some text into the text field
10. Click publish
11. Click on the wrench icon and then click 'Export/Import'
12. Choose 'All for the Range' and then click export
13. Go to 'Sites' under 'Portal' in the Control Panel menu
14. Click Add > Blank Site
15. Put 'Test' as the name and then click save
16. Make sure the control panel switched to the 'Test' site and then click on Documents and Media
17. Click the wrench icon and then click on 'Import/Export'
18. Click on the Import tab
19. Choose the file that was just exported and downloaded
20. Choose 'Copy as New' as the Data Strategy and then click import
The attached log file shows the stacktrace.