Update interface definition error

When I run update interface definition I am getting this dialog box message
Title: Update Definition
Msg: Failed to update interface: [java.lang.NullPointerException]

I can create a new project from the same wsdl and new schemas with no problem. The only thing that has changed is the one schema.
The problem is when I clone the testsuite into the new project from the old, I try to use update interface to make sure that the testcases match the changes to the schema, and I get this error above.
I can work around it, but I'd like to know why this is happening.

Thanks,

Nathan Manning