Classes attribute in admin.TabularInline

[Joao]

I noticed this when using grappelli, but can confirm that the same happens with grappelli disabled and a clean set of static files.
When using the 'classes' attribute in admin.TabularInline I get the following error:
TypeError at xx/xx/xx
sequence item 0: expected string or Unicode, tuple found

I get the same error if I use a 'list' or 'None' instead of a tuple (with corresponding 'list found' and NoneType ending), which is strange since the default value for classes should be 'None'.
Both django's and grappelli's docs ask for a tuple or list, but I get no errors and everything works as expected if I simply comment out the classes line, or give it a string.

[Tim Graham]

There isn't much detail in the report to help with reproducing the issue. What's a minimal example admin.py to reproduce?