set_relationship returns 500 in v7.11. Is backward compatibility not supported?

My use case is simple. I'm creating a Contact, I'm creating a Case and I associate that Case with the contact. When we initially built the integration, only api v4_1 was available.

When use API Key of SugarCRM v7.11, Contact creation and Case creation are succeeding. However, when the method `set_relationship` is invoked, the SugarCRM server is returning a 500.

The same code works for v6.5

How do you suggest we alter the integration to support all versions of SugarCRM for our use case?