How to avoid redundant catching exceptions of mocked methods which can throws exceptions according interface

I have following code in my unit test:

 when(workspaceMock.getNamespaceRegistry()).thenReturn(namespaceRegistryMock);

method getNamespaceRegistry can throws IOException

In terms of java mentioned row is method call and I should to catch it.

But on another hand I understand that this method cannot throw exception because I set this method behaviour there. And it is looks redundant to catch exception here.

Can you clarify this situation and advise best practises?